[CQ-Contest] Re: [TowerTalk] 80m Dipole ssb/cw switching

[Tom Rauch]

> A number of years ago, K7GCO tried to popularize the concept of the
> "single knob tuner" for such applications.  Basically, you cut the 80M
> dipole below the band, then insert a transmitting variable capacitor
> in series with the coax in the shack.  The capacitor is used to null
> out the capacitive reactance exhibited at the operating frequency -
> the higher you go in the band, the capacitance is reduced so the
> transmitter sees a pure resistance.

I spent some time modelling that idea, and using a Smith chart it 
really doesn't work very well...at least as he described the concept. 
The SWR does not improve that much, and the voltages are 
beyond the limits of a receiving variable and it requires over 1000pF 
of capacitance. The major problem is the feedline between the 
antenna and the tuning cap is only 1/2 wl at one frequency, and so 
it is an impedance transformation problem.  

As I recall the SWR was well over 4:1 on the upper end of the band 
with a dipole cut for 3500 kHz with the capacitor optimized. You 
would have to tune the dipole at the low end of 80 meters, or use a 
series L/C system to add inductive reactance to move something 
lower. A series L/C system I carefully looked at was also 
somewhat useless, apparently the whole concept is unworkable. 

The most simple system is just to build a "fat" dipole. If you fan the 
ends of a dipole out several feet with a spreader, it is quite easy to 
cover the CW and SSB areas without any type of mess. A cage 
dipole would also work.

If you are willing to tolerate some loss, Frank Witt AI1H has a 
ARRL Antenna Handbook article on a broadband dipole with 
multiple stubs.

I measured a coaxial dipole and found it was virtually identical to a 
thick dipole, and stubs model very poorly. Obviously the Handbook 
and others are correct when they say simple 50 ohm stubs do not 
cancel significant amounts of reactance. I measured virtually 
identical results to systems described in the ARRL Antenna 
Handbook, with real antennas I built.



  
73, Tom W8JI
W8JI at contesting.com 


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