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SWR and metal roof

Mar 15th 2015, 23:47

KO0Y

Joined: Apr 7th 2012, 19:22
Total Topics: 0
Total Posts: 0
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.
Mar 16th 2015, 05:23

WA9WVX

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
Hello Tim,

I have only a little experience with metal roofs in the Midwest, in and around the Chicago area on commercial buildings but I've never encountered the VSWR changing with Snowfall or moisture which makes me wonder if there is moisture inside of your vertical antenna or if your vertical mast pipe is touching the metal roof setting up some strange bi-metal impedance, sort of a Diode action. If the roof is steel and the mast pipe is aluminum this could happen.

The other strange thing about your VSWR at the antenna and in the ham shack points to the length of your LMR-400 as it could be either too short or too long in the electrical wavelength. I've seen this happen at repeater sites when the feedline is a little bit short and a small jumper has to be made or by adding a couple adaptors to make the feedline appear at the correct length electrically not physically.

If you know the exact physical length of the LMR-400 (measured in Feet) equals 246 divided by Frequency times the coaxial Velocity Factor which is .85 this will give you the electrical length in Quarter wavelengths. A lot of hams do not realize that the Center Conductor of LMR-400 or 9913 is a Solid Piece of Wire making it non-flexible and if for some strange reason it developed a break inside of the feedline that could cause even a bigger problem. Make sure you don't have any 90 degree bends in the feedline as this is NOT recommended.

I've been recommending to use LMR-400UF (Ultra Flexible) which has a Stranded Center Conductor. Lets hope this isn't part of your antenna VSWR problem.

Dan
WA9WVX
Mar 16th 2015, 14:29

KO0Y

Joined: Apr 7th 2012, 19:22
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Total Posts: 0
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?
Mar 16th 2015, 17:56

WA0CBW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
In the two-way radio world what you describe is a classic case of water in the feedline. The fact that the SWR is low between the antenna and the feedline but high at the radio end would indicate a coax problem or a bad connector at the antenna end. If the metal roof or mast was a problem then the SWR would have been bad when measured at the antenna. Many times water in the coax is hard to detect unless you cut off a a few feet and examine the center connector.
Bill
Mar 17th 2015, 03:33

WA9WVX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi Tim,

I would stay away from using a plastic sleeve on the antenna system since the mast pipe and roof are made of steel. Okay about changing the antennas but still having the same problem which leads me and Bill WA0CBW to believe the problem lies in the feedline, even pinhole penetration through the outer jacket could have happened which in turn allowed moisture to creep in undetectable by the human eye.

Your guess at the 9913 being 55 to 60' feet is ONLY an estimate and I'd be only guessing as to what to add to the transmission line this is why you need to have the EXACT length.

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 did a little research on Bill WA0CBW, he's a pretty sharp individual as he taught electronics, then became a Project Manager for Motorola Cellular & the Land Mobile Two-Way Radio Sector and ended up a Motorola Service Shop performing the same tasks before retiring (Lucky Him).

I spent my 34 year career working for Motorola in Schaumburg, Illinois wore many different hats during my career, engineering technician in a corporate R & D lab, consultant for the International Engineering Department, spent time out in the field implementing simulcast radio paging systems, a couple of years in the infrastructure cellular department, working for the Midwest and Hi-Tech depots doing repairs on the high power RF amplifiers and Securenet product line, a few years for the iDEN / Nextel Digital Infrastructure systems implementing MSOs around the U.S. & Canada and ending my career in the Secure Design Center handling Infrastructure SP Bid & Quotes, Design & Development of new configurations on Base Stations and Portable Products, Maintenance of the Production line for Secure Products - Crypto technology and doing Export Control Assignments for the Secure Products before being relieved of my responsibilities in June, 2004. I don't have a positive word towards Motorola or Motorola-Solutions, Inc.

Dan
Mar 17th 2015, 14:31

KO0Y

Joined: Apr 7th 2012, 19:22
Total Topics: 0
Total Posts: 0
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.

Dan


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.

MHz SWR
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?

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