Metal Roof and Base Antenna

[donniefitz2]

Hello, all. I have a unique situation for my base station setup and wondered if anyone could offer some advice. 

In the past I ran a Sirio CX 455 antenna. It works really well for GMRS and I believe it's based on a JPole design (not requiring a ground plane). But, it's limited to higher frequency UHF. Again, perfect for GMRS and the SWR is 1.01 on 467 and 1.02 on 462. I'm also running 35' of LMR 400 with N connectors and the antenna is about 19' high. 

Here's the interesting part: my roof is made of metal tiles. Coated, galvanized steal to be exact and they are not bonded or grounded. I know, weird, and this is what makes things a bit complicated. 

Since buying the Sirio antenna, I got my technicians license and wanted to upgrade to a dual band antenna. Many local radio nerds rave about the Diamond X50 so I purchased an X50NA at HRO. I understand it's not ideal for GMRS, but I think the SWR compromise is worth it. 

So, I mounted the X50 on my mast, which is metal and grounded, standing about 2' off the roof, almost at the peak of the roof. I climbed down the latter and went into my office to check the SWR. It was about 3.5 on the GMRS bands. I was expecting something like 1.6 or 1.8 which is typical of this antenna. Rechecked my connections, tested and re-tested. Still, the SWR was very high. 

That's when it dawned on me that the metal roof is probably causing my problem and acting as a ground plane. I went back on the roof, removed the ground radials from the X50 and checked the SWR. It was much better, about 2.1 for GMRS. So I moved the antenna down (closer to the roof) and the SWR dropped to 1.02 on GMRS. This made the 70cm SWR high though. Eventually I found the sweet spot where 70cm was where it should be and 467 was at 1.6 SWR. VHF SWR is near perfect BTW.

The one thing I noticed and thought was strange: the X50 doesn't hear as well as the Sirio did. The X50 measures lower on the S meter for repeaters that would normally be higher (actually, all of them). All transmit reports are good.

Is it possible that removing the ground radials effects the receive signal? Is there some way I could better use the metal roof as a counterpoise? is the reduced receive signal just a result of this antenna not being tuned specifically for 462/467?

Below is a picture of the roof. The antenna in the picture is not the X50, but the Sirio.

[Sshannon]

4 hours ago, donniefitz2 said:

So, I mounted the X50 on my mast, which is metal and grounded, standing about 2' off the roof, almost at the peak of the roof. I climbed down the latter and went into my office to check the SWR. It was about 3.5 on the GMRS bands. I was expecting something like 1.6 or 1.8 which is typical of this antenna. Rechecked my connections, tested and re-tested. Still, the SWR was very high. 

At this point, right after you mounted it at the same height at the Sirio, and before you removed the counterpoise, what was the SWR on 2 m and 70 cm?

This is a perfect example of everything affecting everything when it comes to antennas, isn’t it?  I would probably put it back the way it was designed, with counterpoise and additional height, and see if it receives better on GMRS even though you would not want to transmit into that 3.5 swr. 

I’m also curious what a sweep looks like at the different heights and three different bands and with and without radials.

i think your loss of receive sensitivity might be due to the effect of lowering the antenna and the resulting change in field. You may be better off using the two separate antennas for the bands they were designed for, x50 for 2 m and 70 cm and the Sirio for GMRS.

I really am interested and will be following this. 

[Sshannon]

P.S. I found a short book that describes the effect of the ground plane and antenna height above the ground plane.  https://nvlpubs.nist.gov/nistpubs/Legacy/IR/nbsir79-1605.pdf

Here’s an excerpt:

“When the antenna height is replaced by 2H, we again obtain the same result as that given in eq (5). This implies that the field strength measured by a receiving antenna at a height of 2H meters above an imperfect earth (a is finite) is approximately 6 decibels more than that measured by the same antenna at one-half of the previous height, when the distance to the transmitter is very much greater than H. This difference in measurement results is solely due to changes in the transmitted field strength by the imperfect earth.”

Also this earlier thread here on the forum discusses a similar question: 

 

Edited March 22, 2022 by Sshannon
Duplicate post and added reference

[donniefitz2]

@Sshannon Thanks for the suggestions. I'll have to read up on that article and understand ground planes better. As for your question about the SWR on 70cm and 2m before I removed the radials, I don't know where it was at. At the time, I didn't have my ham license and was only concerned with getting it working with GMRS. But, once I removed the radials, the antenna immediately improved and started getting the SWR that I was expecting. 

My guess is that if I could get it higher off of the roof, there will be a point where the metal roof wouldn't have as much of an effect on it. But, I really don't want to go higher with it because the mast I have wouldn't support much more length without some re-engineering and although I don't have a HOA, I also don't want to piss off the neighbors. 

I did some reading last night on a ham forum and some guys where suggesting grounding the antenna to the roof itself using copper straps in the case of a metal roof. I'm not sure if that would improve anything but I also don't think it would hurt anything to try either. I'm going to read that PDF and possibly experiment with roof grounding and see what happens. I wish I had an analyzer. 

[axorlov]

I have Diamond X50NA (with N connector) on metal mast 17 feet up from my roof. 7 feet below the X50 there is a TV yagi antenna. Mast is grounded, and shield of the LMR400 is also grounded. My SWR is 1.9 on 462MHz. I also did a quick dirty experiment for the common mode current on the shield: measure SWR when holding cable connector in my hand and when not touching it. Both measurements were 1.9, which indicates none or negligible common mode current.

[donniefitz2]

Yeah, mine is also the NA. The cool thing is, I get an SWR of 1.6 - 1.8 which I think is a great compromise since this is a dual band antenna. The issue is the low receive signal compared to my other (much smaller) antenna. I'm getting ready to get up on the roof today and try a few things. 

[Sshannon]

The General rule of thumb is one half wavelength above the ground, but a large conductive surface like a roof can act like a reflector causing other problems. I wish you the best and I’m curious to see what you learn today. 

[donniefitz2]

41 minutes ago, Sshannon said:

The General rule of thumb is one half wavelength above the ground, but a large conductive surface like a roof can act like a reflector causing other problems. I wish you the best and I’m curious to see what you learn today. 

Figuring things out is what makes this fun. I'll report back tonight. Hopefully I'll have made some kind of progress. 

[Sshannon]

20 minutes ago, donniefitz2 said:

Figuring things out is what makes this fun. I'll report back tonight. Hopefully I'll have made some kind of progress. 

While you’re doing that I plan on hanging an end fed half wave antenna from my eaves and then connecting a NanoVNA to see how it looks. 

[donniefitz2]

Well, after about 10 trips up to the roof and then back down to my office to check SWRs, I have the Diamond X50NA about as dialed in as I will ever get it. 

My SWR on 467 channels was the hardest part to get right. For whatever reason, that frequency is really finicky on this antenna (with a metal roof). I started by just re-mounting the antenna the way it should be, at the top of the mast with the ground radials. 70cm and 2m frequencies where fine. 467 was 3.9 SWR. I took the radials off and it dropped to 2.9. Better but not good. 

After re-positioning, moving it up and down and doing anything else I could think of, I finally found out what caused the high SWR. The mounting brackets for the antenna. I decided to move the upper mounting bracket down a few inches, away from where the antenna starts, and the SWR dropped to 1.8 on 467 and was still good on the other bands. I moved it down another inch and was able to get the SWR to 1.6. Still good on the other bands. 

That's when I tightened everything up, came down and tested the receive. It worked great. Not sure what was going on with the previous position I had it in, but it's all working great now. The metal roof does complicate things, but with enough determination, it works. Time to leave it alone.

[Sshannon]

That’s outstanding!  Everything affects everything, right?

[donniefitz2]

1 minute ago, Sshannon said:

That’s outstanding!  Everything affects everything, right?

Yep. Everything adds up, that's for sure.