From: pfluegerm@gtephx.UUCP (Mike Pflueger)
Subject: Re: Handheld antenna and otherwise
Organization: AG Communication Systems - Phoenix, AZ
Date: Wed, 29 Jul 1992 20:55:13 GMT

In article <1992Jul24.034915.9862@cs.brown.edu>, ST402421@brownvm.brown.edu (Chris Paine) writes:
> better than rubber duckys on handheld radios.  So I made some
> measurments:  What follows are data on one 1976 PA28-161, with two
> Collins VHF 250 comm radios, the associated airframe-mounted antennae,
> and one STS 7600VOR handheld transciver (orphaned).
> 
> I procured the use of a Standing-Wave Ratio (SWR) meter [Swan Electronic
> model SWR-1], and measured the VSWR of each transmitter into each of the
> three antennae and into a 50 ohm (resistive) termination:  data follow:
> 
      [ measurements deleted]

> What does it all mean?  Well, I'll take a stab at interpretation:
> 
> 1)  The all transmitters have approx. constant output impedance.
>     All are spec'ed at 50 ohms; possibly my resistive termination
>     (just a 5W power resistor soldered to a BNC) has some reactance
>     in the RF;  this would explain the SWR > 1 in all cases.
> 
> 2)  The rubber ducky is good only at the lower end of the VHF band.
>     Not too surprising when you consider its just a coil of wire.
>     Also, it cannot handle the higher power (~20 W) of the VHF 250s.
> 
> 3)  The installed antennae are quite good:  The worst they do is a
>     VSWR of 1.5, which is a coupling efficiency of 96%.
> 
> So it appears that coupling into one of the airframe-mounted antennae
> (or having a third installed) would be quite beneficial to handheld
> operation.

Sorry if someone has responded to this already, I'm a little behind in
my news.

Well, contrary to popular belief, the big problem is not with SWR
(within reason) but antenna loss (antenna efficiency).  You often see
this referred to as antenna gain, measured in dB (decibels).  Gain
implies that you're comparing something against something else.  With
antennas, one of the more common comparisons is against a quarter-wave
dipole, a simple antenna that's easy to build in the real world.

To keep it short and avoid getting technical, the com (and vor nav)
antennas you typically find on a plane are essentially quarter wave
dipoles.  Thus, they have 0 dB or no gain as compared to a quarter
wave dipole.  Rubber duckys are short helical antennas which have
about a -6dB gain as compared to a quarter wave antenna.

Each 3db difference is double (or half) the power.  Negative gain
means loss.  So a rubber ducky only radiates 1/4 of the power that a
quarter wave antenna does.  Likewise, the "capture area" of the
antenna is smaller, so it also doesn't receive as well.

So one watt into a quarter wave antenna would have to be 4 watts into
a rubber ducky to get the same range.

Antenna loss is bad, you can see why a simple antenna change can mean
more (and often be cheaper than) increasing transmitter power or
receiver sensitivity.

If you want to have a better antenna, get a telescoping whip and a BNC
connector and build yourself a quarter wave antenna.  Just pull it out
to about 22 1/8" when you want to use it (that tunes it for 127 MHz,
the middle of the VHF aviation band).  23 1/2" would tune it for
120MHz, the middle of the most commonly used freqs.  Only problem is
that's tough to use inside a plane.

-- 
Mike Pflueger, AG Communication Systems, Phoenix, AZ      PP-ASEL
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