[TowerTalk] 80m small antenna

[K7GCO@aol.com]

In a message dated 11/21/00 6:29:35 AM Pacific Standard Time, 
ValErwin@aol.com writes:
<< Chris:
 
 Back in my single life, I lived in various apartments and tried virtually 
 every type approach on a limited space antenna.   I would recommend trying 
 the following schemes:
 
 1. Use an antenna tuner and load the rain gutter -- it works!! (Best!)
 2. Try to find one of the old "Slinky" dipole antennas w/ladder line. 
 (requires tuner)
 3. Use two 80M Hamsticks back-to-back in dipole configuration.
GL
  Val W5PUT 

    Good idea.  How about a screw driver mobile antenna using the rain gutter 
as a ground.  Try vertical, 45 degrees and horizontal orientation.  It has 
the great advantage of being able to resonate it over the whole band and any 
band.  I've done it.  I've also mounted the screw driver mobile antenna at 
the peak of the roof and run 8 radials down and connect to the rain gutter.  
It does a great job.  You can resonate it over the whole band and any band 
regardless of the size of the radial system.

  Another trick is to have a BC 3 gang variable Xc in a box where the Xc is 
in series with the center coax lead to the whip.  Drive it with a selsyn and 
have a cover box.  The technique is this.  The Rr of the whip will generally 
be below 50 ohms.  If the antenna is made longer it becomes inductive 
reactive and the Rr increases proportionally.  The series variable capacitor 
is adjusted to the same capacitive reactance and leaves the resistive Rr.  
With the right lengthening of the screw driver antenna and the right shack 
adjustable Xc in series at the feedpoint, you can get 50 ohms Rr over the 
whole and any band.  Instant 50 ohms on any band.  How do you like them 
impedance matching apples? 

I'm going to mount a screw driver mobile whip on top of a 30'  3" irrigation 
pipe and use the series Xc at the bottom.  I'll be able to adjust it to 50 
ohms and/or resonance over several bands as a 1/4, 5/8 or 3/4 wave which has 
distinct advantages.  3/4 wave gives higher angle coverage also.  Short 
radial systems tend to lower the Rr of the antenna which you are able to 
compensate for.  If the Rr the antenna is above 50 without the series Xc 
which it can be with a lossy mobile coil (ant some are), it will rise 
slightly when the BC 3 gang Xc (1000 uufd) is inserted.  To short out the Xc, 
I bend some plates over such that at full capacity the selsyn driven Xc is 
shorted--no Xc.  I like a selsyn drive better than a motor drive as it's 
faster, it can be calibrated and no shaft couplings are twisted etc.

There is another advantage of this concept.  You can lengthen the whole 
antenna via the screw driver motor to obtain 100 ohms for matching with a 75 
ohm 1/4 wave matching stub.  The advantage is that the Rr is now 100 ohms and 
it reduces the affect of the ground system loss resistance which is in 
series.  it's a may to maximize an antenna system--for contests. There are 
many other advantages which I will cover in a major article I'm preparing.  
For one band verticals just lengthen the antenna with tubing and use the 
series Xc accordingly for the desired Rr.  Eznec can tell you how much.  This 
is a new concept that solves the problem that most Rr's are below 50 and 
resonant at one frequency with verticals.  With a single series Xc fixed or 
variable you can create the desired Rr anywhere in the band.  I gave you some 
food for technical thought.  Use your imagination for all the applications.  
I can hardly count them all.  This is a great concept for mobile also and 
many have thanked me for telling them of it.  New antenna ideas stimulate the 
creative blood for more new ideas. K7GCO  


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