whip antenae

[Russ]

any feedback re these type antenaes installed in lue of conventional aerials to a microair 760.

 

getting plus and minus reports.....( more plus tho )

 

cheers..........

 

 

[Dieselten]

As long as it is resonant and impedance-matched to the feedline it will work just fine. Ideally a whip antenna should be tuned for maximum RF-field (measured using a near-field RF signal-strength meter) which means it is as close to resonant as it will get, then impedance-matched by using a reactive stub to cancel out the reactive component of the measured RF impedance so the antenna now appears as a purely resistive load matching the characteristic impedance of the source and transmission-line, as required by the Maximum Power Transfer theorem.

 

This is what should happen in an ideal world. Unfortunately we live in the real world, so what is done is to deliberately tune the whip antenna out of resonance by trimming the length so the impedance-matching looks good on a reflectometer. (the infamous "low SWR" requirement). We deliberately sacrifice radiated field-strength for a good impedance match, basically because it is easy to cut bits off to get the SWR down, and it is a pain-in-the-posterior to get out the Smith Charts, calculate the resistive and reactive components of the impedance, then design and build a short-circuit stub to perform the impedance-matching function, after the antenna has been cut to a resonant-length. This works well for fixed-frequency transmitters operating at high power output (broadcast stations, for example) but is harder to organise for a transmitter that ranges over a range of frequencies and operates at relativly low powers (precisely what our aircraft radios do).

 

Convenience over-rides technical correctness, partly because with our intermittent, low-power output, it really doesn't matter all that much.

 

In other words, a properly tuned and matched whip antenna will work just fine with any aircraft radio, no matter who makes it. Failing that, a whip antenna for the aviation-band, correctly installed and tuned for a reasonable match to the feed-line, will do just fine. The only issue I have with whip antennas is they add parasitic drag. I prefer a blade, or an embedded dipole, especially in GRP composite machines.

 

 

[jetboy]

Look for antennas spec'd as broadband (usually 108-136 MHz with a VSWR of 2.0:1 or less).

 

D&M C63-1/A is a good example and the way they achieve this is a 12 ohm carbon resistor encapsulated in the base in series with the cable connector. As Dieselten says a small loss of power or signal is not a factor in operation of the radio, these radios and headsets are quite susceptible to bad match of the antenna, problems also may knock off your GPS reception too.

 

Straight wire whip antennas are cheaper but limitations above make them more troublesome with multi avionics installations. Just use RG58c cable NOT foam low loss from the TV installer, you need the loss to control the SWR at the radio with a whip antenna.

 

Ralph

 

 

[Russ]

Interesting info here............thanks.

 

Checking we are on the same page..............."whip" antenaes that i am refering to are not the "wire rod" type.

 

Couple of aero suppliers are promoting "whip antenaes" as a install anywhere.........it is actually about the same physical size as a pak of cigs. SWR is around 2.0

 

Fitting wire type, with that flexy black ground plane thingo attached underside just looks plain ugly on me build

 

cheers........russ