20m FD Dipole
 

Hi Sal,

On Saturday, 25 June 2011 04:16:42 UTC+10, Sal M. Onella wrote:
....
being the most promising, I think. With the help of EZNEC (evaluation
version), I modeled a promising 20m dipole at 30'. It's almost exactly
75 ohms resistive at the freqs I want to use and I'm going to feed it
with RG-6, 0.7 dB/100 feet. My radio has an internal tuner to match
the 75-ohm cable, if I need it.

That sounds fine as far as it goes.

You might consider a 1:1 current balun, a W2DU style one should suit this application fine.

I applaud your efforts to understand the thing using a modelling tool. To some of us, the inquiring mind and the interest in the technology of radio communications is what ham radio is really about. For those not so interested there are contests!

Your model though contains a guess for ground parameters, and possibly other site effects not included. You must expect some variation from model, but those variations would not cause very high VSWR on the line, line losses should be quite acceptable, and an ATU should transform the load it sees to that required for rated output from the transmitter with quite acceptable losses.

In my experience, antennas less dependent on ground are more likely to give consistent performance for field operations where you usually cannot afford to be installing significant ground systems.

Well done OM.

Owen

20m FD Dipole
 

On 6/25/2011 5:28 PM, Owen Duffy wrote:
Hi Sal,

On Saturday, 25 June 2011 04:16:42 UTC+10, Sal M. Onella wrote:
...
being the most promising, I think. With the help of EZNEC (evaluation
version), I modeled a promising 20m dipole at 30'. It's almost exactly
75 ohms resistive at the freqs I want to use and I'm going to feed it
with RG-6, 0.7 dB/100 feet. My radio has an internal tuner to match
the 75-ohm cable, if I need it.

That sounds fine as far as it goes.

You might consider a 1:1 current balun, a W2DU style one should suit this application fine.

I applaud your efforts to understand the thing using a modelling tool. To some of us, the inquiring mind and the interest in the technology of radio communications is what ham radio is really about. For those not so interested there are contests!

Your model though contains a guess for ground parameters, and possibly other site effects not included. You must expect some variation from model, but those variations would not cause very high VSWR on the line, line losses should be quite acceptable, and an ATU should transform the load it sees to that required for rated output from the transmitter with quite acceptable losses.

In my experience, antennas less dependent on ground are more likely to give consistent performance for field operations where you usually cannot afford to be installing significant ground systems.

Well done OM.

Owen

Hi, Owen -

Allow me to jump in here and ask: which antennas are less dependent on
ground?

John - KD5YI

20m FD Dipole
 

On Jun 25, 3:28*pm, Owen Duffy wrote:
Hi Sal,

On Saturday, 25 June 2011 04:16:42 UTC+10, Sal M. Onella *wrote:
...

being the most promising, I think. *With the help of EZNEC (evaluation
version), I modeled a promising 20m dipole at 30'. It's almost exactly
75 ohms resistive at the freqs I want to use and I'm going to feed it
with RG-6, 0.7 dB/100 feet. My radio has an internal tuner to match
the 75-ohm cable, if I need it.

That sounds fine as far as it goes.

You might consider a 1:1 current balun, a W2DU style one should suit this application fine.


Owen

Thanks. I did have the current balun but failed to mention it. I had
25 Mix 43 beads, Coil Winding Specialist P/N SB-6873-43. I see now
(my after-the-fact research) that Mix 73 would have been more
effective at 20m. ("We get too soon old and too late smart.")

I have baluns on all my dipoles, two voltage baluns and two eight-turn
coax cable wound baluns on my home station and a spare eight-turn
balun in my travel box.

I have read that the W2DU balun should be repeated at intervals along
the feedline, some wavelength fraction, but I found no confirmation in
ten minutes of searching. Got anything on that?

"Sal"
(KD6VKW)