Why Does This Work? #Antennas

Woody Phillips - KJ7YYI

OK. I built this and I understand most of it. But it has one characteristic I cannot wrap my head around. Anyone have any ideas?

Here is the (too long) story:

I wanted an NVIS antenna that would work on 80 Meters and if I could get 160 meters, and anything else, that would be great. The reason for wanting an NVIS was so I could speak to, and HEAR, stations here in Arizona. The reason for 80 Meters (75 Meters really) was because that is where the RACES and AETN traffic hangs out (3.990MHz and 3.986 MHz respectively).

After doing a bunch of research I discovered this: 

160 Meters through 6 Meters at less than 2:1 (so no tuner) sounded perfect. In my case I built it without the house (ran the wires zigzag through the trees on either side of my house) and terminated the wires as shown - 1000 Ohm 500 Watt resister. I fed it with 30 feet of decent coax through a 16:1 transformer per the instructions. Transformer and Resistor came from Palomar Engineers. The 400 feet of wire (200 feet per leg) is electric lamp cord that I split down the middle. I used this because I wanted something robust and low resistance. The whole thing is never less than 10 feet or more than 13 feet off the ground.

Believe it or not it works GREAT as advertised. I am well under 2:1 on all bands.

I have had all sorts of people tell me that this will never work, it is just a could warmer, etc. I don't listen to those folks as NONE of my antennas "should" work and they all work GREAT for their intended purpose.

I can run up to 500 watts (the rating of the resistor) through this antenna and it works really well out to a few hundred miles. Since it is low to the ground and is a horizontal antenna, it is also very quiet with S0 noise levels from 20 meters up through 6 Meters and reasonably low on 40, 80 and 160 meters.

Now ... here is the part I do NOT understand and would like opinions about.

I was on the Daily Propagation net the last few days and just for grins I thought I would see how the "NVIS" antenna did. A few times in the past I accidently forgot to switch antennas and made some fairly long contacts on it and wanted to see if that was a fluke.

I received good (55 and 44 reports) from PEI in eastern Canada, Hawaii and New Zealand and pretty much everywhere in between.

I tried this on multiple days and the same result happened. In fact, according to my Log Notes, if I can hear a station anywhere in the world at a signal strength of 5 or above, they will be able to hear me.

So WHY? Everything I have read says this cannot happen with this type of antenna. Note that I never run more than 500 watts and often only 100 watts (today for example) with the same result.

Why does this work???

Bruce Johnson - N7DDT

Short answers: NVIS antennas are close to the ground. NVIS antennas have a high take off angle and are therefore considered to be short range antennas (one hop) . NVIS antennas work best at the frequencies below about 12 MHZ. So 80, 60 and 40 meters are used by EMCOMM groups for that reason, plus antennas are easy to erect and work with at low heights. 
Antenna take off angle is determined by its height above ground. Best DX angles are produced by an antenna higher than 1/2 wavelength above ground (multiple hops).
With an antenna at low height for 40 meters (less than 1/2 wavelength, or less than about 60 feet) it will perform as an NVIS antenna. However, as you go up in frequency, the height above ground now starts to become closer to 1/2 wavelength and therefore a lower take off angle. So, that NVIS antenna at 40 meters is now a DX antenna for the higher frequencies!

Woody Phillips - KJ7YYI

Best answer I have heard so far and makes perfect sense. Also, add in that my house is almost 6000 feet above sea level and .... pretty good antenna at higher frequencies.

Thanks Bruce.

Bruce Johnson - N7DDT

Here is another take on the NVIS story. This is copied from a book named "Receiving Antennas for the Radio Amateur" ARRL Eric P. Nichols KL7AJ.

On Sat, Jan 7, 2023 at 12:24 PM Woody Phillips - KJ7YYI <tekboss@...> wrote:
Best answer I have heard so far and makes perfect sense. Also, add in that my house is almost 6000 feet above sea level and .... pretty good antenna at higher frequencies.

Thanks Bruce.

Steve Hersey - K7OFG

Thanks, Bruce, for sharing that with us. Very interesting reading.
And like Woody's antenna, one never really knows what might work, so don't be afraid to try something out of the ordinary, it just might work better than you think.
Steve, k7ofg