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

Tom Rauch W8JI at contesting.com
Thu Nov 15 05:58:59 EST 2001

> I'm surprised at those results.  The 80M dipoles I've modeled seemed
> to have R values that were reasonably well behaved over a wide range,
> although admittedly not 50 ohms.  In that sense it always seemed like
> simply adding some reactance of the opposite sign would make the
> antenna usable.

This is an excellent example of what happens if you ignore a 
transmission line in a system, and assume a 1/2 wl line is 1/2 wl 
at any frequency in a band.

This idea does work well if you do the compensation at the antenna 
terminals! Heathkit sold a device that did that, it was a remote-
tuned capacitor.

The reactance compensation requires a reactance that ranges from 
inductive..through zero...and to capacitive as you move from below, 
past the resonant frequency, and upwards in frequency. With only 
a capacitor and only 1000pF maximum you have to set the system 
so it is resonant well below 80 meters. The antenna gets pretty 
long at 3.9 or 4 MHz, but the actual problem is caused by the 
transmission line between the capacitor and the antenna. Once 
you stick a mismatched feedline in the system (GCO suggests 
450 ohm line) you not only have increased reactance problems with 
frequency change, you have an impedance transformation and a Q 
increase caused by the mismatched transmission line. Remember 
the transmission line is ONLY 1/2 wl long at one frequency, and 
the useful frequency range (Q) becomes especially problematic if 
you use a higher multiple of 1/2 wl for line length.  A simple 
calculation of line current and reactance indicates voltage can get 
quite high across the capacitor as you move up in frequency, so 
the suggested receiving variables are unworkable except at QRP.

> I don't doubt that the voltages would get rather high, but I figured
> that some HD doorknobs and maybe a vacuum variable would make it work.

They won't. You wind up with what amount to being an antenna 
tuner by the time you are done. 

It would be faster and cheaper to just make a fast antenna.
> Also, it always bothered me that the antenna was actually resonant
> below 3500.  I figured to use the 3650-centered dipole, then add L or
> C as necessary.  But your results would seem to show otherwise.

That's a much better idea, and *may* be workable if you use a 
somewhat matched transmission line rather than the 450 ohm line 
suggested. You'd have to look at a Smith Chart and see what 
happens at the other end of a low-Z line that is 1/2 wl at a specific 
frequency in the band. It would work for sure if you changed line 
lengths with frequency, but then consider what you are really 
doing. You are building a two control antenna tuner out of switched 
coax and a variable capacitor! Might as well just use a tuner, any 
length of feedline you like, and save all the work and coax!

I'd bet that is why almost everyone uses a tuner, instead of the 
novel but unworkable idea of a series cap.

73, Tom W8JI
W8JI at contesting.com 

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