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

k8cc k8cc at mediaone.net
Wed Nov 14 17:30:24 EST 2001

At 12:01 PM 11/14/01 -0500, Mike Brown K9MI wrote:
>Another efficient method is feeding the 1/2 wave dipole with open wire and
>using a Johnson Matchbox. This gives maximum frequency flexibility.
> > 73,
> > Tim K3LR
>I just went to this type of setup myself. The thing I need to check
>and see how much bandwidth I get with it before needing to touch
>it up. Of course, this would only come into play durring S&P. My
>dipole ended up being 130ft overall due to our small city lot. Close
>enough I would say though.
>73 - Mike K9MI

For many years, I've used a coax-fed dipole cut for 3650 KHz, which I've 
matched to 1:1 on both CW and phone with a MN-2000 tuner.  This is very 
convenient, but recently I've come to wonder about the overall system 
bandwidth.  All of the K8CC amplifiers use glass bottle tubes such as 
3-1000Z, 3-500Z, 4-1000A etc. (OK, my one 3CX1200 is not glass, but 
electrically its a 3-1000Z) which are not too cranky about load impedance 

I recently ran some informal experiments with my setup.  I looked at the 
2:1 SWR bandwidth at 3650 KHz (tuner out of line) vs. 3525 (tuner in line) 
and (3825 tuner in line).  I found that the bandwidth at 3825 was only 
slightly narrower than at 3650, while the bandwidth at 3525 was 
significantly narrower.

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.

Its been my experience with shunt feed verticals on 160 that series 
elements (L or C) tend not to narrow the bandwidth of the system, however 
shunt elements will when they form part of a matching network.  The 80M 
dipole CW vs. SSB requires a single-element series solution such as Jim 
Breakall has described.  Using a full "tuner" when changes to the resistive 
part are not required, will probably narrow the system SWR bandwidth.



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