NVIS Antenna Modifications


Latest update: July 12, 2012
Pat Lambert - W0IPL

Overview

The following are the E-mails I received about modifications to existing NVIS Antenna designs.

Enhancements to Existing 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.

From AK0B

A NVIS type we have used at field day the last several years is from a 73 mag in 1959 I think. It is the SGA (Super Gain Antenna).

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. ?

Stan AK0B
Ak0b at swbell dot net

From OZ3XO

Hi Pat..

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

From K0SSI

Hi Pat,
Last year, we assisted WBØDSG, Don, with a simulated RACES emergency at the Telluride airport. I used a 34' section of 14 guage wire approximately 4 1/2 feet off the ground horizontal attached directly to my Icom AH-4 coupler which was mounted on the rear of my SUV, with approximately the same length counterpoise (afterthought) on the ground the oposite direction! Had no problem being heard anywhere in Colorado by anyone listening (State EOC, etc.). We were also lucky that 75 stayed in for the duration of the exercise! I'm going to be experimenting with a new type of antenna utilizing a 22' aluminum telescoping mast with the 3 guywires being part of the antenna with 3 counterpoise wires. This will be loading the aluminum mast at its base and radiating vertically AND inverted 3 legged V with a tuner or coupler.

73
Bill
KØSSI

From W5ZIT

On 07/12/2012 05:06 PM, Jim Brown wrote:
Here is an anecdote for an NVIS situation I experienced when I was a new ham around 1953. I managed to get a 75 meter end fed Zepp up about 60 feet between two trees. I was used to working stations around 50-100 miles away on a long wire (350 ft) about 20 feet off the ground and was disappointed that the new Zepp performed so much worse than the long wire.

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