Given the unique nature of your particular fuselage, don't rely on increasing be 1/4 lengths of Lambda (one complete wavelength - at 72MHz ~4 meters).
What will dictate successful (or not) application will be the length outside the insulator, the carbon fuselage length itself (the fuselage length is is most likely not 1/4 lambda).
Head out to the open area you have chosen that is free of fences, powerlines, buildings etc., set your aircraft on a non-conductive surface at least 1/4 wavelength off the ground (not critical) with say 1/4 wavelength of extra antenna sticking out the back. Use a plastic tube so it is straight off the back of the fuse (important - use drinking straws).
The reason for this is to minimize as many variables while tuning, keeping it straight helps this. Now have someone walk directly away from the tail of the aircraft holding the TX (with the antenna totally collapsed) looking for maximum range, moving a single control (eg: RUDD). With the both antenna elements in the same plane (pointing at each other), this offers probably the worst (at least what we can achieve on the ground) range. The advantage of having someone else wlaking with the TX, is you get to see, and more importantly hear whats going on at the aircraft.
At the point where the RX no longer reliably responds (failsafe, whatever), have your helper walk back "just" enough to regain control.
Now cut off 6" of antenna (and the tube) and repeat and look for adequate control. Repaeat the range check to see if you gained range. If you did repeat and cut off another 6" a the new limit of range. continue until you lose range, than add back the last 6". You can repeat and add back only 3", but your results may be too hard to discern.
The whole idea is to tune the antenna length to maximum range. MAke sure that each time your helper walks away (backwards - looking at you) he takes the very same path away from the aircraft. The idea is to do everything precisely the same way for each test to have meaningful results.
You can remove the plastic tube now. You may be surprised at just how much antenna is actually floating freely behind the aircraft when you are done...
Tom Watson wrote:
Well now,
In fiddling with a 72MHz RX in a particular plane (Pike), there were only two solutions that gave an adequate ground range-check:
1. Stock length antenna dangling straight out of the cabin, 90 degrees to the fuse. Quite impractical. Any other solution involving the standard antenna length gave frighteningly short range.
2. Exactly doubling the antenna length, exiting the fuse halfway down the boom, taped to the bottom of the boom all the way back, leaving the excess dangling. Invisible from more than six feet away and glitch-free ranging to vision limits.
I had read earlier that as long as one kept the antenna in multiples of its own length, it should work.
Tom
[EMAIL PROTECTED] wrote:
Excellent post! Lots of good information, Doug.
Here's a minor nit-pick:
(Now, if you add another 1/4 wavelength, or a multiple of that, you'll probably slightly improve range. But only slightly.)
I'll bet you were really talking about adding another 1/2 wavelength or
multiple of that. Adding another 1/4 wavelength would create a 1/2 wave
end-fed antenna, which would have a much higher input impedance. On the other
hand, converting a 1/4 wave antenna into a 3/4 wave antenna would make sense.
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