[TowerTalk] 80m DX special dipole

[Jim Brown]

On 9/11/2018 12:27 PM, Guido Tedeschi wrote:
> I built an 80m DX special dipole.
>
> http://1vc.typepad.com/ethergeist/2011/01/a-broadband-contest-antenna-for-80m.html 
>
>
> I used the Messi & Paoloni BROAD-PRO 50/C cable instead of RG213 
> (https://messi.it/dati/immagini/BROAD-PRO50-All6_EN.pdf).

 From the antenna description :

"Don't substitute anything other than RG8U for the RG213. The lengths of 
the coax stubs are calculated based on the dielectric properties and 
velocity factors of these cables."   Which is sort of confusing, given 
that RG8 and RG213 are fairly generic descriptions nowadays, but what he 
means is stick with a cable with VF = 0.66.

There is, by the way, another very good way to broadband the match to a 
dipole to cover 3500 - 3900 and keep the SWR below 2:1. EE prof W6NL 
says the idea goes back at least 80 years.

Cut the antenna to about 3675 kHz and verify that it resonates there. 
Connect a half-wave of any good 50 ohm coax to the antenna, then add a 
quarter-wave of a good 75 ohm coax. If the combined length is too long, 
coil up the excess. If you need more coax to get to the rig, add any 
length of 50 ohm coax.

For both sections of coax, carefully verify that they're really a half 
wave at 3675 kHz using an analyzer.  The easy way to do that is to 
connect one end of the half wave coax to the analyzer with the far end 
shorted. When it's a half wave, the analyzer should read a near short (1 
ohm or so). With the quarter-wave section, measure it with the far end 
OPEN, and you should see a near short.

An analysis of how this matching works is shown in

http://k9yc.com/PacificonSmithChart.pdf

beginning around slide #46. Slide #47 shows that for an ideal dipole and 
good coax, loss in the line will be 0.4 dB in the middle of the band and 
less than about 0.7 dB at 3500 and 3900. SWR (at the transmitter end) 
will be about 1.4:1 mid-band and about 1.6-1.7 at the band edges.  Slide 
#51 shows that you can increase the first 50 ohm section to a full 
wavelength, yielding lower SWR at the band edges at the cost of 
increased line loss.

73, Jim K9YC