Best SWR achieved?

Hi Guys

I have a Delta pop, bent antenna for VHF mounted on the belly of my RV6 just before the spar.

I am interested in what the best SWR you have achieved in that location.

I was able to get the following SWR on a test rig at home (bench testing)

118.0 1.39
127.0 1.09
136.0 1.41

I am unable to get the same results on the belly of aircraft approx 1.8 SWR

I have been advised that belly mounted VHF is not acceptable and would always have issues.

*All my testing was done in open area away from any buildings/hangars.
*Ground plane is connected to antenna mounting screws and checked with ohm meter.
*Same coax used in both tests (Dummy load used to confirm coax is not at fault)

I don't see any issue with SWR even at 1.8. I have my VHF com antenna on the belly and recall one airport at one location on the ground where I had a problem. That was from shielding by the wing and fuselage, not SWR. No problems ever in the air.

Larry Pardue N5LP

from the gnc 255 manual "The VSWR should be less than 2:1. A VSWR of 2:1 will cause a drop in output power of approximately 12%. " You're good.

SWR

I have also thought that 1.8 would be ok, but for some reasons I am expierencing "talking in bathroom" effects on a number of frequencies. eg 127.6 and 118.1 all the others are pretty good

This is what my radio supplier has advised me asking the same question on SWR

SWR of 1.8 is not very good. It's just good enough to not blow up the transmitter.
Any reasonable installation should be able to get 1.4 or lower on any frequency.
What are you using as antenna ground plane ? In my experience practically all TX issues I have so far worked on myself (with V6 and V10 radios) are due to incorrect ground planes. These sometimes can lead to interesting resonance effects at certain frequencies ( you can hear this in your headset during TX - the side tone - in mild cases it sounds like you are talking in a bathroom, in very bad cases you get howling, squealing or distorted voice (sometimes only if you speak loudly - which means high carrier modulation).

Anything less than 2:1 is fine. Sounds like a radio problem to me, the vendor obviously prefers to blame the installation rather then his equipment, can't imagine that!

The VSWR on the airplane is less than the bench for a bent whip because the antenna element is bent parallel to the ground plane. I straight (swept) antenna typically performs slightly better.

However, the "bathroom effect" is something else. Mic/headset/radio?

What transmitter you using?

SWR

You can only do so much about SWR with a fixed-tuned (non-adjustable) antenna and no tuner in-line. It's going to have a minimum SWR at resonance (which you should set to be at the middle of the comm band by trimming, if necessary) and higher SWR at the edges. The shape of that SWR curve is determined by the style and lossiness of the antenna (its "Q"), and the actual value of SWR at resonance is determined by the mismatch or ratio between the transmitter output/coax characteristic impedance (typically 50 ohms) and that of the antenna feed point.

A well-build and reasonably-shaped antenna of quarter-wave radiating element over a generous, conductive ground-plane will have half the impedance of a half-wave dipole in free space (72 ohms). If your bent-whip quarter-wave comm is 36 ohms at resonance (it should be, give-or-take a few ohms), you're not going to see 1:1 VSWR with a 50 ohm feed line. I'd have to look up the math, but 1.4:1 sounds very reasonable to me. Starting at 1.4:1 means that the SWR will necessarily be higher at the band edges than it would be if you were able to match to 50 ohms +0j (perfect) at the resonant frequency.

A manufacturer who says his radio will blow up above 1.8 SWR (rather than fold-back the output to protect the final) has no business marketing his product or giving customer support.

Bottom line, IMO, you will not see better than what you currently have without a matching device at the antenna feed point, and you shouldn't need such a thing anyway. (Unless you develop a lossy, corroded connection, which will add resistance and make the SWR look better - at the expense of true performance, which is a whole 'nother story.)

-Stormy

Thanks for the input

Thanks for the input guys

Well this is my DIY testing rig made to confirm the antenna itself was able to achieve a good SWR (Trimmed it about 10mm to get SWR of 1.05 at 118.0 to 1.4 @ 127.0)



The radio is a MGL V6 and the supplier is working very closely with me to resolve the problem (Support has been excellent) but I didn't want to set myself an unachievble goal if SWR of 1.8 was acceptable, hence asking the question to you guys.

I am going to experiment with the antenna on the aircraft and see if I can resolve the problem.

BTW: I used cheap Coax on the rig not to waste money..... RG400 used in the aircraft.

SWR

I haven't slept in a Holiday Inn recently, but why would the vendor have you cut and change from 1.05 to 1.4? Maybe I'm missing something.

What is serving as a ground plane in your test rig?

SWR

Wirejock 1.05 to 1.5 is my SWR range

The ground plane is made up of 4 steel wires (550mm long) connected to a piece of aluminium the antenna is mounted to at the base.

SWR

Ahhh. That makes sense. Duh. Sorry. That range is very good.

Better match on the test rig -

It's because you have drooped your radials away from the radiator, getting closer to 50 ohms impedance for your quarter-wave monopole antenna. As you dress the radials further away fom the radiating element so the they look more and more like the other half of a dipole (180 degrees between the two poles) your feed point impedance will go from 36 to 72 ohms. At a VSWR of 1.0:1, you've found the droop angle where it's 50 ohms. Not so easy to do using the belly of your monocoque fuselage as your ground plane, since the "droop angle" is fixed by Van himself

If you find a spot on the belly where the installation gives 50 ohms and 1:1, it was luck, and time to grab a beer.

-Stormy

I’m surprised the vendor suggested cutting the element to change the VSWR without understanding the antenna construction. The antenna also appears to be stripped of the White Powder Coating on the element, did you purchase the antenna this way?
I would appreciate an email identifying the vendor so I can communicate with him.

Based on this mis-information, I will attempt to clarify the types of VHF aircraft antennas and their construction.

Three types of VHF aircraft antennas are used on general aviation aircraft, the Rod Antenna, Pi-Network Antenna, and the Shorted Stub Antenna.

The simple rod antenna was commonly used in the early days of VHF aircraft communication, the antenna consisted of a bent metal rod with a feed thru insulator allowing the antenna to be mounted on the aircraft belly. The co-ax was stripped and the center conductor was attached to the mounting nut. The co-ax braid was grounded to the aircraft fuselage or ground plane.
With most of the activity in the 118 to 127 MHZ range the antennas frequency response was more than adequate to cover the frequency and still have tolerable VSWR.

As the air traffic increased and more ATC facilities came on line, the frequency range was increased to 118 to 137 MHZ. The simple rod antenna proved unable to cover the wider range with reasonable VSWR performance.

Antenna designers went to work and developed matching networks to increase the antennas frequency response and meet the needs of the new wide band transceivers.

The Pi Network and Shorted Stub designs are widely in use today. Examples of the three types are listed below.

Simple rod antenna

http://www.aircraftspruce.com/pages/av/antenna_com/av534.php

Pi Network Antennas
http://www.aircraftspruce.com/pages/av/antenna_com/av-17.php
http://www.aircraftspruce.com/pages/av/antenna_com/av10_ant.php
http://www.aircraftspruce.com/catalog/avpages/comant121.php
http://www.aircraftspruce.com/catalog/avpages/comant122.php

Shorted Stub Antenna
http://www.aircraftspruce.com/pages/av/antenna_com/dmC63_1.php
http://www.aircraftspruce.com/pages/av/antenna_com/c63_2.php
http://deltapopaviation.com/VHF_Com_Antennas.html




On the Rod antenna, a shorter element length will increase the resonant frequency, adding element length will decrease resonant frequency.
It is not that simple on the antennas with a built in matching network.
Both the Pi Network and the Shorted Stub antenna performance depend on the inductor / shorted stub relationship to the element length, changing one with out changing the other will degrade the wide frequency response of the antenna.
If the intent is to limit the frequency to a very narrow range, cutting the element length may work out but don’t expect good performance at the band edges.

Checking the antenna installation if fine with a VSWR device to insure it is in the spec the manufacture listed. Both Dorne & Margolin and Comant lists VSWR’s of < 3.0 for the bent element and < 2.0 for the straight element antenna.
Changes to the antenna design and network relationship and expecting wide frequency response out to band edges will require much more sophisticated test equipment.

Keep in mind when testing antennas that the Pi Network designs will show open when inserting a Volt Ohm Meter (VOM) across the BNC connector and the Shorted Stub will show a short.

By the way I’m not aware of any bent element VHF Com antennas designed for general aviation VHF Com that offer VSWR’s less than 1.4 over the entire 118 to 137 MHZ range. If anyone is aware of one, I would like to learn about.

Clarification

Hi Guys

Just to clarify
My supplier never asked me to tune my antenna. I also never said they did?

I am getting desperate to get my comm working as flying is not enjoyable the way it is now, so with my lack of knowledge I started experimenting with the antenna because I was advised by the supplier that 1.8SWR is not acceptable and causing my radio issues

They also advised the following
It is unusual to mount the VHF antenna at the bottom of the aircraft (not illegal, just unusual)

SWR of 1.8 is not very good. It's just good enough to not blow up the transmitter.

In my experience practically all TX issues I have so far worked on myself are due to incorrect ground planes


So the way I see it I have checked:

Ground plane ok - metal aircraft with mounting screws making good connection

Coax tested with dummy load and SWR showing 1.03

Antenna tested giving 1.8 SWR at worst

Could it actually be the radio?

AJ85WA said:

Could it actually be the radio?

Click to expand...

Short answer: YES!

I achieved a perfect SWR of 1.00 a while back...

... and then realized my meter was dead....

You hit the jack pot here on the thread. Posters have taken more time than I ever saw....even on a ham radio forum, to illustrate the design and differences of VHF whips for GA. Buy them a beer!
Back to business.... I think you can take your test jig apart. Your plane doesn't have drooping radials. It also will have an element pretty close to earth when you call ground control. Lots of variables will be different. Simply bond the antenna you choose (and save money over Spruce) and buy Delta Pop. Ground bonding, coax fittings and quality wire make the most difference of all.
A modern radio will protect itself from reflected power (which manifests as heat in the final amp transistors). The blowing up phrase is quite misleading.
And while you have been lead down that path, you might have missed root casuses of noise and echos etc. You need to begin at the radio, MIC levels, Side tone levels, A bench test if needed to see if the radio is in alignment.
Even operators have been caught up not knowing that a typical mic, such as a Lightspeed Zulu have a tiny screw and potentiometer on it for gain adjustments. Audio panels that are not set up and leveled properly can induce noise as well. Hight voltage strobe wiring is or used to be a common culprit. LED systems are pretty quiet now days.
Don't sweat a VSWR around 2 much. Look around the system for the bad guys.... that are letting the smoke out.
And buy a quality belly antenna from DP or Spruce. (no affiliation with either)
Nick
Avionics Tech- 41 years ATP/CFII/HELO/SEA/BALLOON/P25 Tactical

flightlogic said:

A modern radio will protect itself from reflected power (which manifests as heat in the final amp transistors). The blowing up phrase is quite misleading.

Click to expand...

Glad for your expertise here. Unfortunately I do not get to review the circuits for the modern radios used in aircraft so I cannot contribute to the discussion. My question is: is this true for the MGL V6 radio? The "blowing up" comment from the manufacturer's represantatnive suggest they may not have such protection.

I agree about the helpful descriptions of the antenna designs.

Another factor with belly mounting an antenna on the tailwheel RV is the proximity of the gearlegs to the antenna. These are very colse to the 1/4 wavelength of the antenna itself so will significantly influence the radiation pattern and possibly the VSWR as well.

an expert in the lab

There is an RF designer/engineer who reads our threads but usually does not post.... to avoid employer conflict of interest etc.
I have seen his radio design work close up for over two decades. He lives and breathes this stuff....
I have permission to copy and paste a bit of his knowledge here:

{{{{{{{Bent whip antennas were designed for bottom mount installations and there are no issues with this aviator using a bent whip antenna as he intends to.



The typical aviation antenna, especially a bent whip can present ~ a 2.25:1 VSWR at the band edges in a fixed wing installation in a bottom mount install.



In fact in a helicopter install, which most comm. radios are also designed and marketed to do double duty, the VSWR in either a bottom mount between the skids or for a top mount near the rotor will see a low VSWR around 2.3:1 at midband frequencies and it will see a band edge VSWR around 3:1 or higher which is still acceptable for the transmitter.



Some additional information on comm. radio designs: Many transmitters are also designed to have the sidetone audio directly sampled off of the reflected power of the transmitters output and this is done so that the sidetone audio will increase in amplitude, almost to the point of becoming distorted and this is normal and is used as an indication of approaching a really really high VSWR level, sort of a audible VSWR indication to the user.



And even in these cases the transmitter is still entirely protected and capable of handling the extremely high VSWR.}}}}}}

Regards, Flightlogic

Posting my results

As the author of this thread, I thought it would be appropriate to post my end results for future readers.

I have removed the entire radio, coax and antenna for testing then re installed it with the same coax and antenna and it now works so clear on the unicom frequency I had problems with before, I have not tested the entire band but it is already sounding soooo great and not sure what the issue was.

Currently my SWR is still between 1.8 and 2.2 with good RX and TX with power output of 5-6 watts (So SWR was not the cause)

I did find a badly bent female centre pin coax connection on the back of the radio which could have caused it, although a dummy load showed swr 1.02

Adjusted my mic setting on my headset to minimum

I have got a radio check in the air from 30nm away 5x5. I am glad the problem is resolved but bit disappointed that i couldn't pinpoint the fault....and that I cut my antenna

Thanks to everyone for replying with great insight to this topic, I am sure we will help out many future readers to come.

AJ