[TowerTalk] resonance/swr/impedance plots

[K7GCO@aol.com]

In a message dated 11/30/00 6:47:31 AM Pacific Standard Time, aa4lr@arrl.net 
writes:
<< 
 This debate has raged on before. The issue isn't resonance at all, but 
 loss. If a 10 foot whip is matched on 75m, the antenna system is 
 resonant. However, it is likely to have lots of losses that a sixty-some 
 foot whip doesn't. If one could design a lossless matching network, the 
 efficiency of the 10 foot whip would approach that of a sixty-some foot 
 whip. If the whips were lossless, then the antennas would act the same. 
 (Conservation of energy -- if there are no losses, then all of the RF 
 energy entering the antenna system must radiate)
 
 There's nothing magic about resonance in the antenna element. The key 
 issue is loss. Eliminate the losses, and it works the same, resonant or 
 not. 
  >>
If you had a lossless matching network and 10' whip, its efficiency should 
allow full raidation of the power fed it and it's efficiency would approach 
that of the full length 1/4 wave on 80M.  However, the vertical lobe of the 
10' whip is fat compared to that of the full 1/4 wave and will not be 
competitive for DX.  So efficiency is not the full picture of performance.  
The 10' whip can do very well locally even with typical matching systems as 
much of it's energy is going into the higher angles.   

I'm planning to use a screw driver mobile whip (with a series BC variable Xc 
driven by a selsyn for Z matching) on my roof with radials to and connected 
to the metal drain gutters and/or dropping to the ground (inside PVC pipe) 
just for the higher angle aspects favoring local performance.  The variable 
inductance features allows resonance anywhere in and/or band.  It can be 
resonated for the BC SW bands also.  Use a coax switch to connect the coax to 
a MFJ Analzyer for quick adjustments of low SWR with no band interferance 
wherever wanted on any frequency.  This tends to compensate for the I Squared 
R losses.  No other vertical has a variable inductance feature.  In the late 
40's Bill Gibson, a W7 from Salem Or had a 10M beam with remotely adjustable 
elements written up in QST.  In contests or general listening with 4 Sq 
antennas or beams for the higher frequencies, it provides a listening antenna 
particularly useful with antennas with good F/B.  I've heard DX stations on 
verticals that I couldn't hear on the back of my beams.  With QRM the 
vertically polarized whip can sometimes favor the desired station on recieve. 
 

A screw driver mobile whip on the roof with radials (don't worry about radial 
resonance-the car isn't) to the gutters can do a great job all by itselt even 
with all it's inherrent losses.  It's adjustability of all the bands is a big 
+ and a longer whip can be used on top along with a horizontal section for 
higher efficiencies and greater bandwidth.  I'd like to have one where the 
drive screw was calibratable with an indicator the shack.  I'm going to talk 
to one of the screw driver mfgs.  Lower efficiency of short (and adjustable) 
verticals can result in other advantages like higher angles of radiation 
which can often be very useful--make use of them.  A 3/4 wave vertical has a 
low and higher angle of radiation and works great.  k7gco

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