Topband: LU5OM shortened dipole (inverted vee) doing a nice job
n4is at comcast.net
Tue Jul 25 13:28:43 EDT 2017
Let me add some comments on Gary and Bruce savvy remarks.
The real question is "how many antennas you have for 160m?"
Using Manuel example, one inverted V and one vertical, for most of us, the answer is two, why not, two feed lines that can be switched on the band switch, feed with two separated coaxial lines. But what about that other 50 MHz Yagi with 120ft of cable, grounded only at the back of the radio, that coaxial shield can be feed too if you use your tuner connected to the shield, so you have another one, ok now it's 3.
Well the right answer is just one system, all these antennas are so close that all interact witch it other as just one, if you feed the Inverted V , the inverted V will feed the tower and the 6m cable, and vice versa. The vertical will never provide any low angle because the inverted V will shoot it energy to the sky.
It is possible to model all these antennas on EZENEC and see the integration. We know that very narrow antennas on VHF can be 5 wave long or more, and the directors 5 wave far from the drive element does interact with the system, on 160m one wave is 240ft (160m), it means that any wire or structure inside that radio is part of your unique irradiation system.
My friend N8PR lives 3 miles from me , my TX tower is 116ft high and Peter's TX tower is 116ft high, my signal used to be 10 db stronger than Pete and both using the same power. We figure out the reason. Peter used to have a 4 square for 80m, on the same tower, 4 x 80m dipoles with a phasing box at the center. Each dipole as a sloper had the lower part connected to the shield of the dipole, the shilled of a 1/4 wave long cable connected to the phasing box, all 4 of them, the 1/4 wave 80m dipole leg is actually 1/8 wave long on 160m, as the same for the feed line, and the same for the other feed line connected on the same box,, when you add 1/8 on for the dipole leg, 1/8 for one feed line and the 1/8 form the other feed line and the 1/8 from the opposite dipole, the result is a 1/2 wave 160m element inside the 160m TX vertical, even with all isolated the integration was so strong that Peter's signal was 10m db bellow comparing with my TX antenna. The system was irradiating UP!! All UP. The 1/2 wave element was working kind of a low dipole.
We just disconnected the 80m, dipole cables from the phasing box and we measured the signal again using RBN and voalahhh.!! Both signals become exactly the same on Peter TX on 160m and my TX on 160m.
Does not matter where are you feeding our 160m antenna, the low SWR does not tell you what you really have irradiating the energy. If you want to compare two antennas at the same place , it is necessary to fiscally remove one when testing the other. Most of the time it is impossible.
One solution is to detune the second antenna at least 20db, 30 db will be better, but hard to achieve.
The same apply to RX antennas, when you have a RX antenna and an inverted V, opening the inverted V, fiscally disconnecting the wires from the coaxial cable at the center of the inverted V, the noise or interaction with you RX antenna can drop 2 or more S unit.
Using google you can find videos from N8PR and PY2XB demonstrating the noise reduction when the TX antenna is detuned (become non resonant on the band you are listening)
The answer for my question is "just one" always one system.
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of K1FZ-Bruce
Sent: Tuesday, July 25, 2017 11:01 AM
To: rxdesign at ssvecnet.com; Topband <topband at contesting.com>
Subject: Re: Topband: LU5OM shortened dipole (inverted vee) doing a nice job
Good information Gary.
Lot of the fun of low band DXing comes from getting new countries, and finding what antenna works best.
Yes, In the transition that takes place at gray line time, there is often high angle taking place.
As in the past, building our own radio from scratch is not so easy, but lets "have at it" with our antennas.
On Tue, 25 Jul 2017 08:21:27 -0400, "StellarCAT" wrote:
that’s a pretty ‘general’ statement! I had a 90’ high inverted L with the bend supported by a tower – it was only about 50’ from a 143’ tower ... it had ~30 100’ radials under it ... and I managed to work 100 countries in 89 consecutive days - from Arizona! That included some pretty rare/distant entities. It worked VERY well as far as I was concerned.
I say this only so that someone reading your comment, having only this as an option, isn’t dissuaded from trying it ... if it is what you have available – go for it!
As for comparing a V at a low height (for most everyone it WILL be at a low height) to a vertical and saying the V was better would, I believe, suggest a feed system issue I’d think on the vertical. I’d think it would beat out a horizontal, for long distance DX, most of the time – and substantially at that. The vertical that is. OR the ground losses are really substantial. Or both.
Just because DX is worked using a low horizontal antenna doesn’t imply something is “good” ... it only implies it is sufficient. “Good”ness is very subjective.
but as they, as we all say – do what you have to ...
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