[Antennas] need help with HF mobile mag mount

2002年10月29日 15:04:52 -0500

I don't believe the 'ground plane' is the issue!! The radiating portion
of the vertical antenna is capacitively coupled to the roof [ground
plane]. The capacitive coupling is the principle means of rf conduction.
The capacitor is in parallel with the dc path.
I've used the Hustler with great success from 10 to 40 meters! [Never
tried it on 80 or 160].
First, get the tuner out of the circuit! Then tune the antenna to
resonance [not minimum VSWR] by adjusting the length of the upper
stinger [use an antenna analyzer]. The Hustler is a shortened antenna
and should have a radiation resistance of 15 to 20 ohms, or a VSWR of
approximately 2.5 to 1. Use a matching coil [LM-1 or equivalent] to
bring the impedance up to 50 ohms. Once matched, at resonance you should
have a bandwidth of 60 to 100 KHz [VSWR 2:1 or less] on 20 meters. Once
this is accomplished you can re-install the antenna tuner for more
bandwidth.
Note: your antenna system must include a base section, the coil
installed above the base, the stinger above the tuning coil. I used the
small spring between the coil and stinger to get some flex into the top
section.
My Hustler installation gave me an antenna about 5 feet in length. The
antenna pattern is vertically polarized. These points are important for
two reasons.
1) You loose about 3 'S'[~ -18 dB] units due to cross polarization when
compared to a horizontal dipole. You loose an additional 'S' unit [~
-5.3 dB on 20 meters] compared to a dipole because of the shortness of
the antenna.
So, if you are comparing the same signal as heard at your home QTH on a
dipole and the same signal in the mobile a change of -3 to -4 'S' units
is typical and normal. Or, if a friend is listening to your signal from
the home QTH and comparing it to the mobile you get the same result. A
mobile antenna on 20 meters is not as efficient as a full size dipole!
In conclusion, all mobile antennas for HF suffer from cross polarization
losses [~ - 18dB] and shortened antenna losses [smaller effective
radiating cross section][~ -5 dB on 20 meters].
Get it to resonance! Then adjust VSWR! Then have fun!!!!!!!
73, Deacon Dave, W1MCE
Manager, Textron Antenna Range, retired
Armin Hachmer wrote:
>> has anyone had good results?
> I am using a hustler 20m element
> on 4 big magmount pads in the middle of a minivan roof. a manual tuner
> reads flat. results are lousy. it hears well but does not get out well. the local radio store told me mag mounts dont
> couple well enough to give a good ground plane.
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