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Odd Comm Problem

tlump51

Member
I am using a Microair 760 radio with a Bob Archer wingtip antenna in my RV4. The radio/antenna combination works fine except at frequencies below 120.5. For any frequency below that I have to be almost on top of the airport before I can be heard. I can usually receive in these frequencies from about twice as far out as I can transmit successfully.
Has anyone else had a similar problem? Any ideas what to check first?
 
First thing I'd check is your SWR (standing wave ratio). There are meters you can buy for that; if you have no idea what I'm talking about, track down your local ham radio club and ask them. Almost certainly you'll find somebody who has one; you might even get lucky and find somebody with an analyzer, which will give you a nice graph of SWR across a range of frequencies.
 
Thanks. I know there is something called SWR associated with antennas, but that's about it. Something new to learn. I will take your advice and see what I can find.
 
A little more info: an antenna is only resonant at one frequency (well, and harmonics, but that's another discussion). If everything is ideal, you'll have a 1:1 SWR at that frequency, and the SWR will get higher the further you stray from that frequency (above or below). The rate at which it climbs varies by antenna design (not germane here). Higher SWR means more power wasted--power that's not being sent out where somebody can receive it.

The fact that your antenna works at the upper end of the band, but falls off at the lower end, suggests that the antenna is tuned too high. That can be caused by a few things. The first, and most basic, is that it's simply too short (smaller things have higher resonant frequencies--think of the pipes on an organ, or of bells). The fix for that would be to add length. Another is that it's close enough to other metal that it's being de-tuned. That's also highly likely, given its proximity to a large metal structure. This is where an antenna analyzer would be really helpful. Think of the analyzer as automating the task of trying the frequency, noting the SWR, stepping up a few kHz, then taking another measurement, etc., and graphing it all:
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You can do it by hand, but it's very time-consuming. With an analyzer, you can run an analysis, reposition the antenna, then run it again. Much quicker, but considerably more expensive than a simple SWR meter (which is why you're more likely to find someone with an SWR meter). I actually have a dual-needle power meter (forward and reflected power), and the scale has SWR markings that you read where the needles cross:
JETSJTWXVUHF-2.jpg



Anyhow, my advice to find a ham club stands. We tend to be very helpful people (especially if you offer a ride in exchange for the help!), love to talk about our hobby, and who knows--you might just decide to become one yourself. Goes great with flying!


Good luck!
 
I am using a Microair 760 radio with a Bob Archer wingtip antenna in my RV4. Any ideas what to check first?

Not an expert or ham operator here but...

The Bob Archer Nav antennas work well in the wingtip because they are intended to be mounted horizontally. The COM antennas work best in a vertical installation, which is very difficult to do with the limited space in the wingtip.

A search here will turn up other threads on this topic and antenna.
 
Lacking any test equipment, you could try lengthening your antenna by about 2? (make sure you have good electrical contact) and test-fly. If the problem at 120 MHz goes away (and maybe now 130 MHz doesn?t work well), the issue is the antenna. BTW, coms use vertical polarization. You want the leading edge of your Archer to have as much vertical run as possible. If it is all horizontal you cannot expect good performance.
 
It looks like this one has pretty much been covered:

* Wingtip-mounted com antennas don't really have enough room for the height of a proper 1/4-wave whip or dipole.

* Com signals, especially with ground stations, work best with vertical polarization.

* It's hard to beat a good old 1/4-wave whip for SWR. Lots of RVs have them on the belly.

Getting into OT territory:

* Between me and the last two owners, we tried about six different internal antennas on my HP-18 sailplane; none of them turned in an SWR much lower than about 3 (very poor). I eventually gave up and installed a 1/4-wave whip.

* On my HP-24 sailplane kit, we build a tuned 1/4-wave dipole into the rudder. Of course, that means the rudder has to be fiberglass or Kevlar in order to be transparent to radio frequencies.
 
Thanks everyone for the advice. Like so many builders, I was loathe to have any antennas in the air stream with my RV. I'm headed for Oshkosh next month and need to feel confident in my radio and I'm in the midst of a panel upgrade. I decided discretion is the better part of valor and I should go with a known commodity. So, I bought a whip antenna from DPAviation. I'm going to give that a try while leaving the coax from the Bob Archer antenna available so I can switch back and forth between the two for comparison later for testing. I'm one of those guys that loves building as much as flying, so I look at the tuning of the wingtip antenna as an opportunity to learn a new subject.
Hopefully, the whip antenna will solve the problem. I will keep you posted.
 
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