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<< Lancair Builders' Mail List >>
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Dave,
Here is something that you absolutely need to validate before you decide
to go on testing different antennae and such. The something that I am
talking about is VSWR (voltage standing wave ratio). One of the most
contributory elements in your system to VSWR is the length of your
antenna cable. If you have a high VSWR, then your signal will be out of
phase at your receiver. The higher the ratio, the higher the out of
phase condition. You can interpret the out of phase term for
resistance. If you have an unusually high VSWR, then the high out of
phase condition can cause a high resistance to the signal.
Consequently, your signal will be very weak when getting to the
receiver. You can validate your VSWR by one of two methods. You can
sweep test the antenna and cable. An antenna sweep is simply taking the
antenna and cable separate from the plane and with test equipment sweep
the band width and allow the equipment to compute the return loss. In
other words, the signal will have some loss as the signal is radiated.
This loss can identify holes in the bandwidth and anomalies that will
indicate trouble spots. The other way is to get Bird meter or a
bidirectional power meter (SWR) and put a signal into the cable and
antenna and then use the meter to indicate how much power is being
returned or bouncing back through the cable. If the return power is
high (or more than 1.5:1, you have a high VSWR. A local ham radio
operator can help you with this more manual test. It is cheaper than
renting the test set that will sweep the antenna and just as effective.
You can change the VSWR of the cable antenna combination by adjusting
the length of the cable. After trial and error you can get a very good
match.
The wings interfering don't make much sense to me because you would have
the same condition in an aluminum plane. But, the recommended position
of your antenna (inside the fuse) may be detrimental to your
performance. Try mounting the antenna external and see how it affects
performance. Or am I interpreting your email incorrectly when you say
'in the compartment behind the baggage area'? If your antenna is
external, then I don't see that your problem is the wings interfering
with the receive signal. If that were the case every spam can would
have the same problem. I'd look more at your antenna cable length.
I don't know if any of this will help.....but since you were looking for
suggestions I thought that i would thow my 2 cents in.
Curtis Krouse N753K
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