MURS base antenna question

[KC1LML]

Yesterday I fine-tuned a mag mount 2m/70cm antenna to MURS for my car. Today I'm thinking about getting another one and mounting it either to my chimney cap or my metal porch roof. The difference in height is abt 14 feet. The chimney cap is stainless steel, so I would have to glue it down or maybe attach a small steel plate on the underside of the cap. Heat shouldn't be a problem. I can position the antenna between the 2 flues. That cap is 25" x 55".

My question is would the additional 14' be worth the trouble? I used to be a "try it and see" kind of guy when I was younger, but at 75, trips up the roof are a bigger deal, plus I have to do it when my wife is shopping

BTW, that 2m/70cm vertical you see is not a very good match to MURS. SWR ranges 1.5 to 2.5 across the MURS frequencies. I sure don't want to adjust it for MURS.

 

[kb5udf]

You would probably get another mile or two, as I eyeball your pic and run the numbers.

Try this or something like it and plug in the numbers:

VHF/UHF Line of Sight Calculator

It goes without saying whether 75 or 25, recruit fellow hams to help.

 

[popnokick]

The higher the better (so chimney if at all possible). Last I knew (and just tested) stainless steel will hold a magnet just fine (basically it's iron). Did you mean the chimney cap is aluminum?

 

[mmckenna]

The higher the better (so chimney if at all possible). Last I knew (and just tested) stainless steel will hold a magnet just fine (basically it's iron). Did you mean the chimney cap is aluminum?

Depends on the exact alloy used. Some will hold a magnet, some will not. Those alloys that do often don't hold a magnet well, which might mean more than one trip up on the roof.
What matters to the magnetic mount antenna is that there is a conductive surface under it to act as the ground plane. Doesn't matter what type of metal it is.

But, yeah, higher is better, as long as the gains from altitude are not negated by the additional feed line losses.

 

[iMONITOR]

The higher the better (so chimney if at all possible). Last I knew (and just tested) stainless steel will hold a magnet just fine (basically it's iron). Did you mean the chimney cap is aluminum?

mmckenna is right as always!

Why don't magnets work on some stainless steels?

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[KC1LML]

I've had the cap off to clean my flues. It's definitely stainless steel, but I know what you mean about different alloys. Our stainless steel refrigerator does hold magnets. I'm going to see if a magnetic antenna sticks to the cap in the morning and will report back. Too hot to get on the roof shingles right now.

I'm in a tough spot elevation wise. I'm at 755' in my backyard plus another 21' up to cap. But everything around me within a 1/4 mile is higher, except for a narrow area along the river both north and south of me that's about 50' lower. I'm not expecting any miracles with MURS. Up to now, I've just been using them handheld around town. Just another way to communicate in case of an emergency.

Fortunately, as far as 70cm, there is a repeater on a 2270' mountain 4 miles from me. So, I can get out of this valley that way.

 

[iMONITOR]

I'm in a tough spot elevation wise. I'm at 755' in my backyard plus another 21' up to cap. But everything around me within a 1/4 mile is higher, except for a narrow area along the river both north and south of me that's about 50' lower. I'm not expecting any miracles with MURS. Up to now, I've just been using them handheld around town. Just another way to communicate in case of an emergency.

None of that should matter much as long as there's nothing obstructing the line of sight.

 

[kb5udf]

Given that you are in a low spot, you may do better then the calculations would indicate from a simple model that assumes flat terrain. In other words, go for it. As long as you are using decent coax, height is your friend.

 

[alcahuete]

If nothing else, I would just want to get it above your roofline. I don't even like where the 2m/70cm is positioned, TBH. The roofing material is different, but I have a Diamond X510 basically configured like yours, with only a couple feet sticking above the roofline, and my tile roof massively affects the signal. I can only get maybe 5-7 miles in the direction opposite my roof, but easily 50+ on simplex the other direction. Might be different with the wood roof.

 

[KC1LML]

The 2m70cm antenna is a bit of an illusion. The tip is actually about 2' above the roofline. Even so, I'm planning on adding a 10' extension to the mast in the next week or so. That should give me better simplex to the west. The only (2) 70cm repeaters I can reach are both about 4 miles to my east, and I already have good signals with them. Unfortunately, no 2m repeaters in my range.

 

[KC1LML]

A couple updates regarding putting a MURS tuned antenna on my roof - I checked the chimney cap with a pickup magnet attached to an extension pole. Didn't want to walk on my asphalt shingles when it's 92 degrees. Anyway, no magnetic attraction at all. I know it's stainless steel, because I paid $350 and installed it myself a few years ago.

I have a flat 4" x 4" electrical junction box cover that I'm thinking of holding up against the underside of the cap and seeing if the antenna magnet locks onto it from the top. Might do that first thing in the morning.

I took time this afternoon to put a magnetic antenna on my metal porch roof and ran some road tests north, east, south, and west from my house. That particular antenna (Nagoya UT-308UV) already had a decent swr (abt .5) on MURS frequencies. I had the one I had shortened (Tram 1185) on my car roof.

I'm using BTECH MURS-V1 radios at 2 watts. I got out to 2 miles north, 3 1/2 miles east, 3 miles south, and 2 1/2 miles west. That's actually better than I expected. These hills around here are heavily forested with tall northern white pines. I get maybe half those distances with the factory rubber duck antennas outside the house and outside the car.

I'm planning on doing the same road tests once I get that Nagoya antenna on my chimney cap. Hope I won't be disappointed after going to the trouble.

 

[vagrant]

Is the WA1UNN 2m amateur repeater in Francestown working? It is around 9.5 miles and north of you around 1880'. Your roof is not in the way toward that repeater and it is listed at the top of the Crotched Mountain ski area.

Francestown, NH

147.060

123.0

WA1UNN

9.5

You were not joking about being in a tough spot elevation wise. Raising your VHF/UHF antenna above your roofline will help to that other UHF repeater to the west near Dublin, but you really are deep in there.

 

[KC1LML]

Is the WA1UNN 2m amateur repeater in Francestown working? It is around 9.5 miles and north of you around 1880'. Your roof is not in the way toward that repeater and it is listed at the top of the Crotched Mountain ski area.

Francestown, NH

147.060

123.0

WA1UNN

9.5

You were not joking about being in a tough spot elevation wise. Raising your VHF/UHF antenna above your roofline will help to that other UHF repeater to the west near Dublin, but you really are deep in there.

You're right. I should be able to reach that 2m repeater over on Crotched Mountain but haven't been able to. I'll probably have to drive over that way and check it out with a portable. The repeater in Dublin must be down. I drove other there last week and got nothing.

I did pick up a hiker on top of Mt Monadnock the other day on 2m simplex loud and clear. That's about 8.5 miles west of me at 3165' elevation - definitely line of sight.

 

[KC1LML]

I decided not to bother putting a magnetic antenna on my chimney cap after trying my MURS radios on my 2m/70cm base antenna. SWR wasn't too bad. I get .5 on the lowest MURS channel and 1.0 on the highest.

That was after raising the antenna shown in the pic another 10 feet. Now the base of the antenna is well over the roof ridge. I can also pick up the 2m repeater on Crotched Mtn (Francestown) now, So that was worth the effort, since I can't reach any other 2m repeaters.