[Antennas] Diminishing Returns

Thu Jun 4 20:26:16 EDT 2009

-----Original Message-----
I own several spectrum analyzers, but I don't drag them out
for antenna testing.
What I do is turn down the RF gain on the HF radio until the
AGC is inactive. Then I connect the audio from the headphone
jack to an HP Multimeter that displays AC volts in decibels.
This is actually quite accurate, as long as you keep the AGC
out of the act, and don't let the audio clip. Use a sensitive
range on the meter. If you can't get a multimeter with
decibels, you can just display AC volts and use a calculator
to convert to decibels.
The audio could be sent over wires or over UHF rig. In the
latter case, it is important to use only a few kHz of deviation
to avoid any effects from the speech clipper. The nonlinearity
of the speech clipper could ruin the accuracy.
I chuckled at your comment about no one using 16 radials. I
have 120 radials under one of my verticals.
Rick N6RK
---------------------------------
True AGC gets in the way of a true "S" meter reading...especially since the
S meter in HF rigs IS fed from the
AGC detector circuit.....personally I like a wide dynamic range device like
a spectrum analyzer...I have 3 myself..
Two Cushmans and one Motorola/Geneal Dynamics R2670B with the P25 option...
As for radials, 99% of the hams wont have more than 16 (lucky to get
4!)...BUT I have had lots of verticals with 120 radials also...REQUIRED in
the broadcast biz.....an AM station is USUALLY required to have 120 BURIED
1/4wavelength radials..EXCEPT in the case of above ground mounted
radials...where only 4-8 are all that are required (HHHHMMMM). A new local
AM just went in with elevated radials...and IT TALKS..of course, MW is more
GROUNDWAVE than HF which is SKYWAVE...
So the ground system is even more important at MW freqs...but then what do I
know?
Chris
WB5ITT
Former Chief Engineer, KDMX/KEGL AND 1190AM Dallas (with the famous 12 tower
nighttime site east of Dallas...one HELL of a broadside pattern!) YEAH I
KNOW verticals and ground planes....all 12 of those towers had 120 radials
EACH under them! :)