Thanks Jim, some good information there. Im suspecting possible corrosion or
just
bad contact with the whip antenna. I also suspect that the aluminum tubing
which used to be the static port hook up might be messing with the Bob Archer
As it is in very close proximity - might even be touching! Im also planning to
try out an Advanced Aircraft Electronics high gain antenna from Aircraft Spruce
which does not require a ground plane and recommended for composite aircraft.
Regards,
Martin
Sent from my iPhone
> On Jan 28, 2020, at 12:07 PM, h&jeuropa <butcher43@att.net> wrote:
>
>
> Hi Martin,
>
> Boy, those are surprising results! I too used a MFJ analyzer with good
> results.
>
> Im assuming when you measured resonate frequency you mean that was the
> frequency
with the lowest SWR.
>
> To start, verify that the frequency calibration of the MFJ is close. Attach
a dummy load (can be 3 150 ohm resistors in parallel or a simple dipole) and
sweep
across the com frequencies while listening on a hand held or your comm radio.
There is no modulation on the MFJ but you should hear something.
>
> If the MFJ calibration is close, as I expect it is, then you need to tune your
antennas.
>
> For the whip on the belly, be sure you have a good ground plane. Bob Nuckolls
does a pretty good job explaining how a dipole works and what constitutes a
ground plane. Have a look at page 13-11 of AeroElectric Connection book for
details. If you have something similar and there is no corrosion and the
connection
to the coax is good (assuming you are using RG58 or RG400 coax and the
connector is properly attached), then you need to trim your whip. It should
start
at about 22 inches long. Try just taking off " to 3/8 and see what difference
that makes. Fortunately your antenna is too long!
>
> Im really surprised at the Bob Archer antenna. Did you test it installed or
just sitting on a table? Remember any metal near it will affect it. This
thread
---From VAF in post 7 has instructions on how to tune a Bob Archer antenna using
a MFJ 259!
>
> http://www.vansairforce.com/community/showthread.php?t=142149&highlight=tuning+bob+archer
>
> It is a bit more complicated since the length will determine the resonant
> frequency
but the gamma matching circuit will determine the SWR. Although the antenna
is new, be sure there is no corrosion where the aluminum bits are riveted
together.
>
> Finally, you can construct a simple dipole per AEC using solid copper house
> wire
(12-14 gauge) and solder RG58 or RG400 coax to it. Don't worry about a balun
or torrids, this is just for testing and education. Just tape the elements
to a piece of wood. Lead the coax at 90 deg to the elements. See what the
MFJ says about it and try trimming to see the effect. You might want to do this
first to get some confidence in what you are doing:)
>
> I hope some of this helps. Probably whoever loaned you the MFJ can assist.
>
> Jim Butcher
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=494497#494497
>
>
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