[TowerTalk] resonance/swr/impedance plots

[K7GCO@aol.com]

In a message dated 12/1/00 9:02:18 AM Pacific Standard Time, w7ti@jps.net 
writes:

<<It's true that the car body isn't resonant, but it IS coupled closely
 to the earth, which the roof radials aren't.  You may find a
 difference, especially on the lower bands (40, 80).
 
 73, Bill W7TI
  >>
Bill:  The smaller the metal mass the lower the Z for a whip.  Being close to 
the ground just makes it a bit larger--not resonant.  We are often led to 
believe it has to be or should be resonant--desirable but not necessary.   It 
will affect the Z of the feedpoint and it may or may not be around 50 or 75 
and it may also be reactive which can be controlled by adjusting the length 
of the screwdriver antenna (this is a great feature).  If the Rtotal is above 
or below 50 or 75 use matching devices and full transfer of power will occur.

I've used various sizes of metal masses including wires to the rain gutters.  
A wire next to the chimney connected to the hot and cold air ducts also 
worked great for a ground for a screw driver whip.  SWR was always less than 
1.5:1 and often lower and good bandwidth.  I did mention as I remember that I 
dropped wires over the roof to the ground in PVC pipes also to aid 75M.  I 
backed up to a water tower and clamped a mobile whip to it and loaded the 
screw driver whip to it.  Actually the whip here was a radial and matching 
device to the "big vertical".  The shield went to the tower and the center 
lead to the whip.  

I also ran a 120' wire to the ground cable coming down a 110 VAC power line 
pole in my back yard.  I connected to it the ground cable 20' above the 
ground rod.  The 2,110 wires and the neutral center ground wire was 5' 
higher.  It was sort of a gamma match and it was resonant at 3.8 MHz and I 
stuck the wire in one slot of my coax switch with a 1.1:1 SWR and fair 
bandwidth.  It was 1.3:1 on 40M--I lucked out.  I used it for receive 
only--of course.  On 160M the 1/4 wave of wire inverted the low Z at the 
connection, to a high Z in the shack and I had to use a L network to match 
it.  

Another great application for the screw driver whip is on top of a beam--mono 
or tribander.  Over very good installation was on top of a 3 element 20M beam 
mounted next to a T-matched DE.  It used the 20M beam mounted on a tower as 
it's ground plane.  Most verticals aren't that high, it had some directivity 
although was a great listening antenna and sometimes compared to and beat the 
beam.  It had a different angles of radiation due to the polarization.  It 
would take stations out of the QRM on receive at times.  If you get RF in the 
shack on the LF's drop a radial from the top of the tower at about 30 degrees 
and trim it for low SWR.  

I have an external receiver jack in the rear of the rig.  I have a coax 
switch in there that connects to my MFJ Analyzer for a quick check of or tune 
up of my antenna tuners and the resonant frequency of the screw driver whip.  
That way I don't have to put a signal on the air to check SWR.  I can 
resonant the screw driver whip in the BC SW Bands real fast also.  It's a 
very useful antenna on the mobile and base mounted on the center of the roof 
also.  You can extend the whip higher and even run a horizontal wire off 
horizontally.  This will give a higher Z and more bandwidth.  Use your 
creative imagination and look for things that are right about a system 
instead of trying to find out all kinds of things wrong with it.  You will 
get some surprises.  

I attached it to a buried and unused oil tank for a ground (equivalent to car 
in the ground--now that's really grounded), extended the whip, it worked 
great and I could tune it anywhere with the reversing switch in the shack.  
I'm going to talk to the mfg and suggest a way to calibrate the internal 
drive screw so I can tell exactly what turn it is on in the car or shack.

I use a variable Xc at feedpoints and in beam reflectors (make it longer if 
used).  It works great in the mobile.  It takes an average of 20 years for a 
new idea to really catch on.  When it does others take credit for it.  I'm 
waiting for the day on TT when some ham trys the series Xc idea in his 
mobile, any antenna feedpoint or reflector and says "Hey GCO that was a great 
idea, why hasn't ham radio been doing this for years--on and on?  I've been 
suggesting it for over 50 years and showed it in an article in CQ 35 years 
ago with a selsyn drive.  About every 2 years a mobile guy comes up and raves 
about it and after 10 minutes I usually say "you buy dinner and you can tell 
me more about it."  I need food in my stomach to listen longer than 10 
minutes.

Mount a BC 3 gang variable (50 cents in a flea market) in a box with it in 
series with the center lead and connect to the mobile feedpoint.  Insulate 
the shaft coming out of the box.  My whip is mounted on top of my Porsche 
roof 3' behind my ear.  I have an extension shaft extending toward the rear 
view mirror.  Resonate the mobile whip at 3.770 MHZ with the Xc shorted 
out--(bend some plates over at full Xc).  Then operate 3.8-3.9 MHz and the Xc 
will have to be tuned over only about 160 degrees.  SWR will start at about 
1.5:1 at 3.8, drop to 1:1 at 3.85 and rise to 1.5:1 at 3.9 MHz.  The Xc has 
to be adjusted every 20 KHz or so for minimum SWR.  The whip Rtotal starts at 
about 35 ohms and rises to about 75 ohms over 100 KHz when the inductive 
reactance is tuned out with the variable Xc.  On loaded whips the Rr 
increases fast with frequency and that's why this is a very effective and 
simple idea.  I get about 130 KHz between the 1.5:1 SWR points on 40M.  It 
works just great on 160M also. 

W6AOA (not sure on the call--Johnson last name) had a mobile whip with a 
circuit that automatically retunes the whip when ever the frequency is 
changed.  I'll be using that on all my antennas. 

Remember the Rr on 75M of an 8' whip is about 1 ohm.  The typical 35 ohms 
Rtotal measured on 75M mobile whips means that you have 34 ohms loss 
resistance in the coil and car body whether it has steel lining or not in the 
tires.  You have about 3% efficiency.  As you operate higher in frequency the 
Rr increases and the Rloss stays the same.  Are you getting the picture?  
This is the only way to raise the Rr inherit of a short whip and the basic 
idea works on yogis also.  It's usable with a 1/4 wave full size whip as it's 
usually around 35-40 ohms. However no mfg uses it.

With screw driver antennas, the variable Xc is initially adjusted for 
obtaining 50 ohms in the center of the band on any band (the Rtotal changes 
from band to band) and then the inductance is adjusted for resonance over the 
band.  Long live the screw driver whip--and the series variable Xc.  Try it.  
Use a MFJ Analyzer on a coax switch to find the resonant point and SWR.  The 
cars capacity to ground is small potatoes.  When parked like out hunting I 
lay out a couple of 1/4 wave radials and extend the whip. 

For verticals use the largest and longest metal mass you can and resonate the 
antenna.  If the Rtotal is less than 50 ohms, use a series Xc to create 50 
ohms.  That's a "one component tuner".  It's really an L network.  The L or 
inductance part is the longer antenna.  Keep it simple.  K7GCO

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com