Antenna querry

Hello all

Two part'er.

1. Has anyone used this inexpensive antenna?
http://www.rami.com/gaa/antenna-info.cfm?pid=30

2. Thinking of putting it in the top of the vert stab. I have seen the VOR antennas put there, but has anyone put their comm ant there?

Thank you,

I can dig around my junk box and see if the one I originally bought for the Val is still there Wade.....

My story is that I put it on the belly, figuring I'd get as good of performance out of it as I did a similar one I had on the top of my Yankee for many years. I then discovered that with the simple ring-terminal connection, I was probably sending more RF back INTO the cabin as I was radiating it outward! Every indicator in the cockpit went nuts when I keyed the Mic. Replaced it with a better antenna with a Coax connection, and all the problems went away. Would it be better on the tip of the tail? Probably less internal interference, but I bet it would still waste a lot of energy.

If I still have it (might have cut it up for something) you can have it.....

Paul

RV8RIVETER said:

Hello all

Two part'er.

1. Has anyone used this inexpensive antenna?
http://www.rami.com/gaa/antenna-info.cfm?pid=30

2. Thinking of putting it in the top of the vert stab. I have seen the VOR antennas put there, but has anyone put their comm ant there?

Thank you,

Click to expand...

Yep, been using that antenna on my RV-6 since '99. It has worked fine on my plane and is mounted on the turtle deck.

Iron's experience with this antenna:

I then discovered that with the simple ring-terminal connection, I was probably sending more RF back INTO the cabin as I was radiating it outward! Every indicator in the cockpit went nuts when I keyed the Mic.

Click to expand...

Sometimes the RMI uMonitor's fuel flow indicator will flake out for a few seconds after a radio transmission but I have no idea if this is due to the antenna. But all in all, the unit has worked very nicely on my plane and local pilots have commented on how strong the radio sounds. Just make sure you keep the connections clean and tight.

That's the same kind of antenna I have on the belly of my -7A. I occasionally have ground control tell me I have a terrible radio until I turn the plane to hopefully unblank the antenna. No one's ever complained in the air.

I always wondered why my dimmed panel lights would go off when I keyed the transmitter at night. Maybe a coax connection to a better antenna would help. Thanks for the heads-up, Paul.

Mike

Keep it short...

That's one connection where the exposed co-ax center conductor and shield connections need to be as short as possible....

If you are having problems, I bet a little metal shielded box around the connections would work wonders.... ...and be an easy retrofit - have it connect to the aircraft skin, not directly to the co-ax.

gil A

I have one located on the underside of the fuselage just aft of the C/S spar area. Works perfectly both on the ground and in the air. No known interference problems. I suspect that on the top of the vertical stabiliser would not provide an adequate ground plane for the antenna.

Fin 9A

Don't scrimp in the antenna

RV8RIVETER said:

Hello all

Two part'er.

1. Has anyone used this inexpensive antenna?
http://www.rami.com/gaa/antenna-info.cfm?pid=30

2. Thinking of putting it in the top of the vert stab. I have seen the VOR antennas put there, but has anyone put their comm ant there? Thank you,

Click to expand...

Two part'er for the price of ONE!

Yea pop for a real COM antenna with a BNC connector. You can find them on eBay sometimes and save a few bucks.

Vert STAB? Wow some negative reasons not to do that are, one a very long (heavy) coax run with associate signal loss; two, there is a poor horz ground plane on top of the Vert Stab. VOR NAV antennas are horizontally polarized so they work on top of a the vert stab. COM antennas are vertically polarized and need a good horizontal GROUND PLANE to perform to maximum.

GROUND PLANE - A flat conductive (aluminum) surface on the fuselage, about 3 feet round is the ground plane (24" square works but bigger is better). Base stations often use a series of horizontal whiskers or spokes from the antenna base for a ground plane.

Polarization means how the waves propagate through the air. Vertical means just that, the waves go up and down, so the orientation of antenna matches the signal, vertical or up and down. If you lay a COM antenna flat or horizontal the signal will virtually disappear. The horz ground plane "reflects" the vertical element, making it act electrically much longer. Our antennas are about +20" long, which equals about 1/4 wave length of our normal freq (118-137 Mhz). A full length antenna would need to be 4 times as long (and draggy). The ground plane is part of the antenna. Composite planes and rag-N-tube planes typically use an aluminum or copper foil ground plane backing up the antenna behind the fabric or composite.


My take for best three locations.

> BEST OVER ALL - On the belly between the firewall and main spar, usually off center, but not always. This gives good performance overall with a super simple short coax run. Also the thick belly skin fwd of main spar makes a solid place to mount the antenna. It also looks best. Its hardly noticed on the parked on the ground, unless you look for it. The interference from the gear legs (trike or TD) seems to be minor to nil. I'm sure the radiant pattern may be slightly affected in some directions but not enough to notice. It also may be hard to get a full three feet recommended spacing to other antennas, but this rule gets violated all the time with little ill effect.

> Top of turtle deck (1/2 way between wing trailing edge and tail) for best ground operations (gnd, tower) and good air performance. Installation is a bit of a pain, primarily the coax run is long.

> Bottom of fuselage opposite of above top mounted antenna. This probably gives best true air performance; its the farthest from the gear legs and out in the open; it's on a good ground plane, and has line of sight to anything on the ground with no structure in the way.

******************************************************************************************

Antenna design and installation is part science and art. I have flown planes with top and bottom antennas connected to the two differnt COM radios. Sometimes I needed to switch radios (antennas) to get better reception on the ground or air. There's no perfect antenna location. Stay away from hidden wing tip antennas. They don't work well compared to an external vertically polarized antenna. When people say hidden wing-tip antennas work, WORK might mean getting the tower 5 miles straight out over the nose. COM is something you need and use all the time. It's a safety item to have a clear COM that can receive and transmit, so get the better antennas and mount them appropriately.

Flying over the jungles of South America or some remote location around the world where radio stations are few and far between, pushing VHF radio range, I'd put the antenna on the lower/aft-mid-fuselage location. I'd also get a 16 watt transmitter. In the US we have RCO (remote controlled outlets) and rarely push VHF range where antenna location makes much difference. The fwd belly location MAY(?) in theory have somewhat a non omni-directional quality due to gear acting as polarizing elements. This directional affect is probably only measurable on an Electromagnetic (EMC) Anechoic Chamber. You may have seen pictures of antenna testing using aircraft models mounted on a poll in a large auditorium sized room, with triangular shapes on the wall.



Military planes are tested in chambers because it works both ways, transmitting as well as being a radar target. They use sensitive instruments to measure the radiation pattern or radar signature. For RV'ers, if you can talk to who you want and hear them, it's good enough. Don't reinvent the wheel and follow the plan is usually the best way to go. When it comes to building, I copy and steal ideas shamelessly. If it works don't fix it. Some times new ideas are good but almost 100% of the time there are trade-offs and no free lunch. There is no perfect antenna location. Don't discount the weight and cost of coax runs. You may want to look into the better RG400, but for short runs RG58 still works and is cheap.

Like Fin stated above, you need the ground plane to be as square to the antenna mast as possible for good performance. I had that same antenna on my old TriPacer. It was mounted on the aluminum upper wing-to-fuselage fairing and performed well even with the old Genave radio.

Steve

VS dipole antenna - Vertical polarization

Your installation would appear to be a standard dipole antenna vertically polarized for COM operation.
The antenna element being the vertical COM antenna and the VS being the vertical COM ground plane.

Just like the two element VOR antenna certified aircraft like to stick in the top of the VS horizontally polarized.

Until I installed a Bob Archer VS COM antenna with a fairing I made, I had a $3 made from parts antenna similar to what you are proposing.

I called it the cloud tickler.

Regards,
Jim Ayers

antenna

I have this cheapo job on my 9A. Haven't flown yet, but the issue is, hard to tighten up securely to the belly skin. Doubler installed,.040 round 4 inches in diameter. So far, testing electronics on the ground, I see no big interference problems, and SWR is good at 1.12:1, but that is on the ground, and reflection from the shop floor might be a factor. All in all, I don't like it because it is hard to mount securely, and am considering a Comant flange bount with a BNC nipple. If I do replace, I will gladly give you this one...
Regards...Chris

I think that is the same antenna that I got from Wag Aero. It's mounted on the belly of my RV-8 off center and aft of the main spar. I've never had any problems communicating with towers or enroute with FSS or Approach. The only problem is it is easy to damage when cleaning or working on the underside of the plane. I recently broke mine off after twice accidentally bumping it when cleaning the plane. It broke off right at the threads. No surprise. The bright side is it is 3/16" stainless steel rod which is available for a couple of bucks from many places. The other bright side is the antenna only costs about $22 from Wag Aero.

Update - I just looked at the picture closer. The antenna you are asking about is more sophisticated than the one I have. It has a tapered whip and better structure around the mount.

Thanks Paul, I appreciate the offer.

Thanks for the replies. I did not look closesly enough at the picture and figured that a RAMI antenna would have a BNC at least, didn't think $80 was skimping.

I know about the ground plane requirements, polarization, ect. But I do not see why the tail would not be a good location. Metal conductive aircraft, the entire aircraft is a ground plane. Now the question seems to be, is the horizontal structure of the tail enough or is the rest of the aircraft whith close enough proximity? F-16's (at least when I worked on them) have the VHF antenna built into the leading edge of the vert stab about 2/3 up, no problems and absolutely no horizontal surface (under the antenna) at all. I remember reading about an RV-4 with LOM engine that used a blade on the tail and had no comm problems, but the fairing increased the tail height enough to cause weather vaning issues.

I guess I will have to get a wattmeter to know for sure. But, please keep the experiences/ideas coming.

It's still not as good

If you look at the VSWR for the cheap antenna its 3 : 1

The better com antennas advertise a 2:1 or even 1.8:1 VSWR. LINK

That means with 3:1 you will have about 24.9% reflected energy back to the radio. Receiving it has a 1.25 dB loss.

With VSWR 2:1 - Transmitting 11% reflected - Receiving 0.5 dB loss

The wider the antenna element, like the blade types, the wider sweat spot verses a rod. There is only one freq that an antenna is idealized for, but the wider the element the more efficient it will be over a wider band of frequencies (band width).

To be honest 14% or 0.75 dB may not make much difference, but as was pointed out the shielded BNC connector is better. Will you notice it? May be? probably not? I'm going to guess drag is a little less with an aerodynamic base than plain rod and donut insulator. Have you seen this kind of antenna on any modern plane built since the 1960's?

az_gila said:

That's one connection where the exposed co-ax center conductor and shield connections need to be as short as possible....

If you are having problems, I bet a little metal shielded box around the connections would work wonders.... ...and be an easy retrofit - have it connect to the aircraft skin, not directly to the co-ax.

gil A

Click to expand...

Thanks for the info, Gil. I can shorten up the connections. What is the nature of the "metal shielded box"? Is is much more than a small aluminum enclosure of some kind?

Thanks,

Mike

gmcjetpilot said:

If you look at the VSWR for the cheap antenna its 3 : 1

The better com antennas advertise a 2:1 or even 1.8:1 VSWR. LINK

Click to expand...

George, did you look at the drag value for the link you gave, 3lbs is alot more than 0.56lbs? I wish Comant would give drag values although it is splitting hairs so to speak.

The "high dollar" Comant antennas run in the 2.5:1 with their bent whip almost everybody uses is at 3:1 so I am afraid I don't see much difference there except cost.

Not worried about the base so much, since it will be under the fairing. I agree totally about the BNC as I didn't see that until the second poster pointed that out.

Coupla thoughts

Just about any metalic object can be made to radiate RF. I have transmitted (HAM) using the gutters on my house! Antenna design can be counterintuitive to the unschooled, which includes me. Stick with a commercial design.

One big problem with the primitive wire rod antenna is a lack of impedance matching between the 50 ohm coaxial cable and the antenna rod. A commercial antenna will have a matching network so that the RF coming down the feedline (or vice versa on receive) will not be reflected back into the transmitter. Without a matching network a good share of the transmitter's RF will bounce back from the antenna and radiate into the cockpit. The feedline becomes an unintended radiator and RF sensitive aircraft instruments go nuts.

A low SWR (standing wave ratio) is no guarantee of good antenna performance. For example, a dummy antenna is a purely resistive load with a very low SWR (desirable) used for tuning up transmitters. It radiates hardly at all so that RF interference is avoided while working on the transmitter. Great SWR but no radiation. The name of the game is to excite the ether and not warm up the coax and connectors.

I think that a HAM with antenna knowledge and antenna analyzer instruments (common with HAMS) could design a vertical antenna mounted on top of the fin that would work well. If that doesn't describe you I'd advise sticking with what has been shown to work. That's what I'm going to do. Good luck.

Roger Roger hair split

RV8RIVETER said:

George, did you look at the drag value for the link you gave, 3lbs is a lot more than 0.56lbs? I wish Comant would give drag values although it is splitting hairs so to speak.

The "high dollar" Comant antennas run in the 2.5:1 with their bent whip almost everybody uses is at 3:1 so I am afraid I don't see much difference there except cost.

Not worried about the base so much, since it will be under the fairing. I agree totally about the BNC as I didn't see that until the second poster pointed that out.

Click to expand...

Did not notice. I see that they drag numbers for 350 mph, cool. However drag is exponential with speed, so 350 to 200 is like 66% less drag. So the 3.43 lbs drag is about 1 lb drag or 3/4 hp, which is about 0.25 mph.

You have to compare BENT whip to BENT whip. Frontal area is where the drag is. Also the better performance is probably due to the antenna being more vertically polarized.

3.43 lbs drag is about right. The fiberglass rod may be larger and more important, more of the whip is not bent and is sticking up, with full frontal area into the breeze.

The BENT whip version with sealed base and BNC connector is only 0.66 lbs @ 350 mph, per their specs, compared to the 0.56 lbs. At 200 mph that would be about 1/4 lb drag. That sounds too small drag wise to be realistic.

I did some mad scientist math and came up with a normal or generic conservative 1 lb of drag at 200 mph for a typical bent whip (about 10" facing the breeze), with round 20" long whip (3/16" dia) on aerodynamic base. At 350 miles it would be about 3.12 lbs drag.

Any way splitting hairs. Good stuff. For a $60,000 plane an extra $150 for something you use every flight seems value added, but I've got no dog in the fight, just spouting opinion. I don't think drag is an issue for any bent whip. There all about the same.

One thing that is obvious is the 1/4 mph drag is so small that internal antennas are kind of a questionable advantage for loss of performance. If the COM is 1/3 mph and VOR 2/3 mph you are talking about 1 mph total at high speed cruise.

Sorry George, no fight intened, just couldn't help myself poking a little fun at you.

Steve. Thanks. I am not going to build/design an antenna, but just trying to get some feedback on some of the antennas out there. I am an FCC with multiple endorsements and 20yr avionics/elect tech, and you are right building a simple antenna is not that difficult. But it is not worth the time and effort to try and build an effecient antenna. My grandfather and Uncle where electrical engineers and Ham radio guys, guess that is where the electron bug bit me.

RVJim thansk for the info. I assume your Bob Archer vert stab antenna works fine?

Little boxes...

alcladrv said:

Thanks for the info, Gil. I can shorten up the connections. What is the nature of the "metal shielded box"? Is is much more than a small aluminum enclosure of some kind?

Thanks,

Mike

Click to expand...

That's exactly it... a small aluminum enclosure, could be split into two and held together with self-tapping screws.

You might be able to get a little aluminum box at any electronics store... just make sure any surface finish is conductive...

This Radio Shack one is a little large at 5 inches, but you can get the idea...



gil A

Thanks, Gil

Mike

Won't help

az_gila said:

That's exactly it... a small aluminum enclosure, could be split into two and held together with self-tapping screws.

You might be able to get a little aluminum box at any electronics store... just make sure any surface finish is conductive...

This Radio Shack one is a little large at 5 inches, but you can get the idea...

gil A

Click to expand...

Sorry Gil, I have to disagree. The bulk of the problem from stray RF in the cockpit (causing lights and instruments to go wonky while transmitting) is almost certainly caused by the entire coaxial feedline radiating, not just the little stub of center conductor that is exposed at the antenna. The reason for this is high SWR in the feedline. The RF is reflected back from the antenna due to an impedance mismatch at the antenna. RF is actually radiating from the entire external shield of the coax. Covering up the antenna connection with a small aluminum box will avail ye naught. RF is mysterious and strange, but this is what it does. Ref: post #16

While I'm warmed to the subject, allow me to speculate that the reason a primitive wire rod antenna worked so well in the past is two-fold. First, a vertical antenna working against a perpendicular ground plane (typical airplane com antenna deal) has a characteristic impedance pretty close to 50 ohms, hence no special matching network was required and a direct connection to the 50 ohm coaxial feedline was close enough.

Close enough for tube transmitters, that is. The second reason is that old-timey tube transmitters can tolerate a higher SWR than modern transistor transmitters. In fact, in order to protect the final power transistors, all modern transmitters incorporate what is called a fold-back circuit. The fold-back circuit senses high SWR and severely limits power in the final amplifier to protect the power transistors. A husky 16 watt transmitter, working into a badly matched antenna, may be limited to less than a watt. And that miserable watt has to fight its way out past that same impedance mismatch to get to the antenna. No wonder no one can hear it. That's my theory. For the radio cognoscenti out there this is elementary, but I am sure it is news to many.

Specs are specs.....

Stephen Lindberg said:

Sorry Gil, I have to disagree. The bulk of the problem from stray RF in the cockpit (causing lights and instruments to go wonky while transmitting) is almost certainly caused by the entire coaxial feedline radiating, not just the little stub of center conductor that is exposed at the antenna. The reason for this is high SWR in the feedline. The RF is reflected back from the antenna due to an impedance mismatch at the antenna. RF is actually radiating from the entire external shield of the coax. Covering up the antenna connection with a small aluminum box will avail ye naught. RF is mysterious and strange, but this is what it does. Ref: post #16

While I'm warmed to the subject, allow me to speculate that the reason a primitive wire rod antenna worked so well in the past is two-fold. First, a vertical antenna working against a perpendicular ground plane (typical airplane com antenna deal) has a characteristic impedance pretty close to 50 ohms, hence no special matching network was required and a direct connection to the 50 ohm coaxial feedline was close enough.

Close enough for tube transmitters, that is. The second reason is that old-timey tube transmitters can tolerate a higher SWR than modern transistor transmitters. In fact, in order to protect the final power transistors, all modern transmitters incorporate what is called a fold-back circuit. The fold-back circuit senses high SWR and severely limits power in the final amplifier to protect the power transistors. A husky 16 watt transmitter, working into a badly matched antenna, may be limited to less than a watt. And that miserable watt has to fight its way out past that same impedance mismatch to get to the antenna. No wonder no one can hear it. That's my theory. For the radio cognoscenti out there this is elementary, but I am sure it is news to many.

Click to expand...

And I'm going to disagree too....

RAMI makes co-ax connected low profile antennae that also has a 3:1 SWR rating... their AV-17 is one.

http://www.rami.com/gaa/antenna-info.cfm?pid=20

As long as the subject AV-534 is correctly installed with short leads to the co-ax, it should also meet the manufacturer's specifications with the same 3:1 max SWR specification as the fancy co-ax connected one.

According to your theory, the expen$ive AV-17 RAMI antenna would also radiate RF all over the place from it's co-ax shield, since it has the same SWR specification.... RAMI says both antennae are suitable for aircraft - and both meet the FAA TSO requirements. Note that a TSO comm. antenna connected to a TSO comm. radio makes a TSO comm. system when connected by co-ax meeting the comm. manufacturer's requirements...

Looking through various King, Garmin and Narco late model Tx radio specs, I can't find a new transmitter (not the vacuum tube ones you refer to) that has a quoted max. SWR specification for the comm. antenna.

Garmin does have a mention that 3:1 SWR may loose 50% of the power output, nowhere near the 16 to 1 ratio you speculate above. A 2:1 SWR will also loose a measureable amount of power.

My box idea was a suggestion - along with shortening the co-ax terminating leads - for an existing installation which was troublesome. A low cost, easy to implement idea for an existing problem.

I still stand by that idea, and already have one of the RAMI AV-534 antenna destined for my RV.

We can agree BNC connector is far superior

This all started about this AV-534 "coat hanger" COM antenna: LINK

FACTS (at least to me)

We all can agree that a coax BNC shielded connector of a AV-17 is superior to the crimp lug deal of the AV-534 "coat hanger".

I have to be honest, a solid core RGU-58 coax, stripping it bare, crimping it to a plan-ol lug terminal, screwed onto a threaded post to a coat hanger....... IS CRUDE! (sorry its true)

The AV-17 has a reliable BNC connection that will likely give you a lifetime of trouble free operations. The AV-534 will likely give you problems down the road. No airplane current manufacture uses the "coat hanger" antenna. I have heard of AV-534 types having subtle issues with corrosion and failed connections over time. Remember you are not transmitting AMPS through this stuff you are talking millivolts.

The AV-534 is cheap for a reason, its a FREAKING COAT HANGER with Ben Franklin technology insulators.

The AV-534 was cool before coax & connectors where invented, but we now have the technology of low loss coax and connectors. If the AV-534 NEEDS a box over the connections to keep you from hearing radio signals in your fillings, its not for an airplane. A BNC connector is way better than bare exposed wire and does not need a BOX over it. BTW if making an EMF shield, I'd use copper foil.

[Humor NOTE: The above is 1/2 sarcastic, 1/2 opinion and 1/2 facts (again humor). Please don't be offended if you own a AV-534 antenna or cheapness turns you on. I'm a recovering cheap-O-holic myself, which my girlfriend is helping me with. She has me on a 12 step program for extreme cheap-scape-ness. I buy her 12 shoes and/or purses and I feel better so I'm told. Filling-up the tanks on a RV can cost $200, which is more than an antenna. My Mom told me penny wise, pound foolish. You'll like a AV-17 (or equiv of other bands) which has low drag and a low loss reliable connection, which will not give you problems. Also it looks better and has a more stable base. The AV-534 connection is not reliable or efficient, and I'm not being funny here.]

This is cheap

Make it your self. here is a photo of mine part way done, easy and costs about $4 from a stainless car antenna. There are length calculators on line.
and done

Facts... sort of...

gmcjetpilot said:

...
FACTS (at least to me)

-Coax to BNC plug has less loss than crimp terminal on stripped bare solid coax core wire
-Crimping lug terminals onto coax shield and solid core is crude and hideous!
-Crimp lug on SOLID RGU-58 core is a fatigue break waiting to happen
-BNC connector - Expect 100% lifetime reliability (crimp-on a solid wire not likely)
-An EMF shield is a moot point with a BNC connector​

We all can agree that a coax BNC shielded connector of a AV-17 is superior to the crimp lug deal of the AV-534 "coat hanger".
......

Click to expand...

George... I think you are overdoing it on this one, and I can't determine which part of your message falls into which part of "The above is 1/2 sarcastic, 1/2 opinion and 1/2 facts "....

We can agree the BNC connector version may be better, but it doesn't make the wire version (coat hangar is a bit of a liberty - do you have stainless steel tapered coat hangers......)

-Coax to BNC plug has less loss than crimp terminal on stripped bare solid coax core wire

Yes... but our frequencies with a max. of 138 MHz are pretty easy for any wiring... generic Mil-Spec shielding allows 2 inch pigtails at these frequencies... but, of course we should be as short as possible for a RF lead. No one can tell me what the actual loss is... I bet it's not very large, but this fact is missing.

-Crimping lug terminals onto coax shield and solid core is crude and hideous!

Your opinion - it's been done this way for a long time - 1970's technology used these antennae (including the factory supplied Tiger belly antennae) and they are still working, even with newer, updated radios.
But... don't be hideous, use stranded core RG-58 co-ax....
My personal preference is not to use any solid cored co-ax in an aircraft. It is easy to obtain RG-58 with a stranded core - RG58A/U - Belden 8219 or 8259 is one make... RG-400 appears to be stranded as in the descriptions from the usual aircraft suppliers.

-Crimp lug on SOLID RGU-58 core is a fatigue break waiting to happen

Don't use solid core cable... see above... Provide support just like any other cable end.... including a BNC cable...

-BNC connector - Expect 100% lifetime reliability (crimp-on a solid wire not likely)

Again, I disagree... yes for a professionally installed connector... but I've seen a lot of poorly installed BNC connectors. When assembled, they are almost impossible to inspect. Again, don't use solid core co-ax....
How many of our builders use co-ax connectors specially designed for the newer RG-400 cable? ...as opposed to a "generic" co-ax connector from the radio store?

-An EMF shield is a moot point with a BNC connector

I agree on this one... but again, my box idea was a suggestion for a particular installation that has a problem.... I'd like to hear a report on the problem after the leads have been shortened...

We all can agree that a coax BNC shielded connector of a AV-17 is superior to the crimp lug deal of the AV-534 "coat hanger".

Again, yes... but the manufacturer has tested both and given them both an identical 3:1 SWR specification... which was the original point of my previous response...

gil A

No more postings from me on this subject, unless to answer specific questions...

Apples and oranges

az_gila said:

And I'm going to disagree too....


According to your theory, the expen$ive AV-17 RAMI antenna would also radiate RF all over the place from it's co-ax shield, since it has the same SWR specification....

Click to expand...

Sorry Gil, didn't mean to imply that at all. A 2:1 or even 3:1 SWR probably won't result in RF getting into sensitive cockpit instruments. My point is that a BNC connector is much less lossy (wasteful of RF) than a connection made by attaching the coax leads directly to the antenna, especially when considered over a period of time when corrosion of the joints and moisture entry into the open coax is considered. The BNC connector was engineered to work at VHF and provide low insertion losses. To RF coming down the feedline, the homemade connection looks like a resistor. (In fairness, the BNC connector does, too, but much less so.) With degradation of the homemade connection will come increasing SWR. All this in spite of the advertising copy. Sorry I wasn't more clear.

Also, I was assuming that good commercial antennas have some type of impedance matching network in the antenna base. Maybe not, I haven't split one apart. A HAM would put one in for sure, and tweak it to a fare thee well, as only an amateur can afford to do. As far as TSO'd antennas go, I wonder if that has more to do with structure and less to do with electrical performance. I haven't read the TSO specs, just speculatin'.

SWR is easy to measure but it says nothing about how well an antenna radiates. It is only a measure of how well power is transfered. Radiation patterns and intensities are much harder to measure but that is what really matters. SWR claims by manufacturers are only a sideshow. In the HAM world antenna manufacturers have been known to peddle nonsense, all because RF is strange and mysterious and they can get away with it, sorta. Would airplane antenna guys do that? Nawww.

Don't doubt that high SWR can shut down a moden transmitter. The fold-back circuit has to be able to protect the final amplifier from a feedline short or from keying the transmitter without a load (antenna disconnected). I picked one watt as a likely number if faced with a high (say 10:1) SWR from a deraded feedline/antenna system, which could happen over time. To those who are experiencing stray RF in the cockpit that is interferring with instruments and lights, look there first. Also, a high SWR episode can cause high voltage nodes in the coaxial feedline and internal arcing, ruining the coax and causing perpetual RFI misery, all without any external signs of damage.

Let me know if the little aluminum box over the antenna/feedline connection reduces RFI. I'll happily eat crow if it does.

Frank K: Cool antenna. How well does it work? Matching network in the base or not?

I didn't mean to cause such a ruckus. I'm just a guy who first flew my -7A 2 1/2 years ago and now have 280 hrs. on it using my SL-30 transmitting through my "coat hangar" antenna. I've yet to receive a complaint from ATC about may airborne transmissions from my monthly IFR and VFR trips to and through several large and busy facilities, including New York, Chicago, and Cleveland centers, and New York and Potomac approach controls.

As far as I'm concerned, it isn't broken, but could be improved maybe by shortening the leads, which I'll probably do when I get some time. Thank you all for your input.

Mike

Frank K: Cool antenna. How well does it work? Matching network in the base or not?[/QUOTE]
No nothing in side, this is based on a plan from Jim Weir a long time ago. It does work great thats what others tell me-, no RF problems. But we should remember there is more to good transmitting than just the antenna
a inside pic.

Just this one time I went too far? ha-ha

az_gila said:

George... I think you are overdoing it on this one....

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Gil, I am always overdoing it. bygones, but when dealing with micro volts, the crimp, lug bare no shield is a bummer from a loss stand point. When new its all good, but I have heard of many radio problems with old planes and this type of antenna. Each time problems where traced back to the antenna, corrosion and poor contacts. Its no big deal, just opinion and its not my plane.

RG-400 appears to be stranded as in the descriptions from the usual aircraft suppliers.

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Not always some RG400 is solid core, some stranded. I'm not worried about solid core coaxial. It's plenty OK in coax with BNC connections. It's not a matter of fatigue or robustness, stranded coax core is about flexibility. I laugh at using coaxial that cost $1.85/ft with an el-chepo antenna, when you can use $0.32/ft (or less) RG58 or the better LMR-195 equiv (about $0.45/ft). It seems ironic. Gil you make all good points, as usual, about good connections.

Again, yes... but the manufacturer has tested both and given them both an identical 3:1 SWR specification... which was the original point of my previous response...

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The SWR is a function of vertical element not being straight up with a BENT WHIP, regardless of connection style. The BENT WHIP is not vertical so its not ideal, regardless of BNC or crimp wire wiring. The lower SWR's are for the ones that are not bent, but they have more drag. If I implied the connector was the reason for the SWR, thats not what I meant. SWR 3:1 is the spec for both, true. I can't get worried about SWR 3:1. SWR is a BASE 10 LOG function, so gain/loss between say SWR 2:1 v 3:1 is no big deal. Your points about good/bad connections are true. You can have a bad BNC connector for sure, which are hard to find. However over the years I just worry about the open wire connection of the "coat hanger" getting corroded.

We are talking micro volts, so just a little corrosion can cause some resistance and changes in antenna performance. The stainless whip (what kind of SS don't know) is OK but in direct contact with dissimilar metals its not ideal. The crimp terminal lug, copper wire, out in the air, touching SS will corrode, ever so slightly. The advantage of a sealed antenna base & BNC, besides fatigue, is less corrosion. The BNC is not going to last forever either, but with the open wiring moisture can get into it. Now add the electical voltage from transmitting, sitting there year after year, with no maintence or inspection, the connection might become poor over time. May be annual disassembly, inspection, cleaning or cap sealing the connection might help. It might take 10 years for corrosion to affect radio performance noticeably? The BNC is isolated some what from the base. It's sits high and dry. Plus the BNC compatible plating is almost inert. It should be more maintence free. That's my story and I'm sticking to it. (We are truly splitting air molecules.)

FrankK90989 said:

No nothing in side, this is based on a plan from Jim Weir a long time ago. It does work great that's what others tell me-, no RF problems. But we should remember there is more to good transmitting than just the antenna

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I have two commercial antennas. You can clearly see the sealed potting, silicon filling the base. Sealing might not be a bad idea (ref my corrosion rant and rave above). It does add weight. You antenna looks cool, nice work. (See thumb nail below, one is made Dorne and Margolin Inc., the other made my Decibel Products, Inc., you see the black or orange potting)
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alcladrv said:

I didn't mean to cause such a ruckus....transmitting through my "coat hangar" antenna. As far as I'm concerned, it isn't broken, but could be improved maybe by shortening the leads, which I'll probably do when I get some time. Thank you all for your input. Mike

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GEE I'm just kidding about the "coat hanger". I say "coat hanger" affectionately. It brings back memories of my childhood beatings. I'm KIDDING Mom & Dad, Love you. Lets call the "coat hanger" by the acronym: A-CRRED - Aerospace Communication Radiation Reception Element Device. Most antennas are just a wire or wires in some kind, shape or configuration. The devil is in the detail. Bad connections are bad connections whether a BNC or crimp lug. The basic physics of the "coat hanger", I mean A-CRRED is good, just the small detail of how you connect it is different. No offense meant. "Roger Roger, 10 by 10, over and out."