The following are the E-mails I received about modifications to existing NVIS Antenna designs.
I think every one of us has come across an antenna design that can be improved upon. This segment is to document those enhancements that have proven to be well worth the time.
It is a dipole that is 7 ft off the ground and has a counterpoise directly under the antenna lying on the ground. We usually use three counterpoise spaced at about six feet.
A folded dipole is a better match than the dipole, but we normally feed the dipole with open wire line. The antenna is cut for 40 meters but works well on 20 and 15.
I would be very much interested in seeing how this antenna turns out when modeled.
I believe it has about 9 db gain straight up. ?
Ak0b at swbell dot net
I built the version of the AS-2259 as described by N6MRX, Dr. Carl O. Jelinek
I found that by replacing the 50 Ohm Coax with a 450 Ohm Feeder and using an ATU at the Ground, it gave me 10-12 db MORE signal on ALL Bands from 160 to 30 Meters.
Next I will try lengthening the wire elements to resonance at 40M and 80M using first the 450 Ohm twin lead then 50 Ohm coax. I plan to feed both with an ATU at the ground.
Vy 73, de OZ3XO - Stig
I decided to put another half wave doublet under the Zepp about 20 feet off the ground and routed the 300 ohm feedline for the doublet back to the shack and put a 300 pF variable capacitor across the end of the 300 Ohm line.
As I tuned the capacitor I could observe a peak/dip on the final loading plate meter. It looked like tuning a discriminator as the 'S' curve was very evident on the plate meter.
I noticed that as I tuned the capacitor the stations I had been working peaked up several S units on the receiver S meter. This was using AM so I had a nice steady signal to look at as I tuned.
With one antenna under the other, I thought I was peaking up a broadside direction and was happy that the 60 ft high Zepp was working as well as the long wire. Now I realize that I was peaking the lower antenna up as a reflector and was directing the signal straight up.
Just a recollection to reinforce the conclusions you are reaching with your NVIS experiments.
73 - Jim W5ZIT
I had a problem with the SWR. 3.1 at resonance. Tried everything. Nothing helped, because it was too close to the ground. (Had to keep it from being seen, with a 6' high brick fence around the property).
Anyway...why am I writing you? I tried inserting an MFJ 909 Impedance matching capacitor type at the feed point (after the balun). It worked! 1.2 SWR on 7180. Perfect. I sealed up the unit so water can't get into it. But, the (near) perfect match was made with this unit. Saves my "finals" too. Thanks again for your article...
Bill Sawders, K7ZM