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Latest Topics

Topic Created Posts Views Last Activity
SWR and metal roof Mar 15th, 23:47 6 98 1 week, 4 days ago
Twin Lead and metal mast? Oct 31st 2014, 16:12 2 300 on 31/10/14
Dipole or loop? Aug 19th 2014, 03:02 11 959 on 5/9/14
dBmicrowatts? Jul 16th 2014, 19:51 3 384 on 17/7/14
Dipole crossing vertical? May 2nd 2014, 00:01 3 432 on 5/5/14
Ground Plane antenna near metal roof? Jul 6th 2013, 15:28 4 1,642 on 9/7/13
Does paint affect a Yagi? Jan 17th 2013, 02:40 4 942 on 25/1/13
50 or 75 ohm reference for analyzer? Jan 6th 2013, 22:52 2 923 on 7/1/13
Using ladder line with Yagi Jan 2nd 2013, 00:58 1 1,011 on 2/1/13
Two new ham questions about antennas Dec 9th 2012, 16:56 7 2,130 on 26/12/12

Latest Posts

Topic Author Posted On
SWR and metal roof KO0Y 1 week, 4 days ago
Quote by WA9WVX

If you had a known good 50 Ohm Dummy Load and disconnected the RF cable from the GP-6, connected it to the 50 Ohm Dummy Load, re-measured the VSWR again and it read high, this would point the problem directly at the transmission line.


I have an oil can dummy load, and I will do that when the weather clears again. Nice yesterday, but foggy today, and I stay off the roof in bad weather; it's 35 feet from peak to ground!

In the meantime, I rigged a signal generator and got these results.

144 >10
145 >10
146 7.0
147 4.5
148 2.5
149 1.7
150 1.1
151 1.1
152 1.7
153 2.3

Resonance has shifted upwards 4 MHz. Cold moisture produce such a neat result?
SWR and metal roof KO0Y 1 week, 5 days ago
Dan, thanks. The coax is the 400MAX from DX Engineering and is about 55-60' long. It is stranded copper inside, and is rated for 2.5" permanent bends (no bends are that small). The antenna during the snowstorm was my GP-3, which I replaced two days ago with a new GP-6. I replaced the connector as the new one uses N, and there was no sign of moisture; it had been well-sealed with silicon tape inside the tubular base and the copper is shiny. At ground level, the AD lightning arrestor had been covered with snow. It was also sealed with silicone, but I have temporarily bypassed it with a new length of coax. So I am pretty sure there is no moisture in the antenna or feed line.

The roof and mast are steel; the mounting bolts are stainless, and the antenna mount is aluminum, so perhaps there is some kind of bi-metal impedance effect as you said. The coax shield is bonded to the roof through the mount, and the mast is grounded with a copper wire to the ground rod, while the coax shield is also grounded at ground level. Could there be a ground loop formed between the coax ground-level and the coax at the mast through the mast's copper wire to ground?

If either, or both of these are in play, then it seems to me I should place a plastic sleeve between the antenna mount and the mast. If, on the other hand, the roof is acting as some kind of additional ground radial (the antenna has three radials at its base), then the recourse would be to raise the antenna higher with a longer mast. This I am loath to do, as we can get high winds here.

The SWR has come down a bit more, but is still over 3.5. I guess I will wait for the rest of the snow to melt (next few days sunny in the 60s) and see if it reduces further. Do you think the plastic sleeve would be a good idea?
SWR and metal roof KO0Y 1 week, 6 days ago
I have a GP-6 2 meter/440 vertical mounted on a metal pipe that is attached to one end of a metal roof. While the SWR measures 1.1-1.5 at the antenna, it has always been higher at the radio. The feed line is LMR400. When we got a lot of snow recently, the SWR went to 8-10 or higher. I thought moisture had gotten in, but I found none when I was able to climb up there, or in any other connection. The SWR has gradually come down as the snow melts. Moreover, on mornings when the temperature has dropped below freezing, SWR is low, only to climb as the sun begins to melt the snow! Could water on the metal roof affect the antenna impedance? The base of the vertical is about five feet above the roof. Would raising it higher help? Thanks for help.
Twin Lead and metal mast? KO0Y on 31/10/14
I bought a dozen of those army surplus four foot aluminum poles and plan to use them to hold up the center feed point of a long non-resonant dipole (260'). Well, it might be more of an inverted v since the ends will be 20-25' off the ground, and the center twice that.

I will use 300 ohm twin lead for the feed line back to a balun in the shack. I know that the feed line should leave the dipole at a right angle. My question is whether I can run the line down the mast, or if it needs to run away at a right angle to both the antenna and the mast? Thanks for advice.
Dipole or loop? KO0Y on 1/9/14
I shortened the feed line by about 14 feet, which did shift the impedance curves upward enough to make the 4.2 MHz region easier to tune. I wish I could attach those graphs here, but doesn't seem possible. Two dipoles would be better, I can see, but that is a project that will have to wait. Thanks again for all comments; I'm beginning to understand this a bit better now.

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