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1-9 of 9 messages
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Page 1 of 1
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groud planes for mag mounts...
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by KC0VCU on October 11, 2005
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I am pretty much given to understand that if you build a ground plane antena, with a radial ground plane, that the impedance and performance is improved by drooping the radials as much as 45 degrees.
Some of us are using mag mount antenas in our appartments during the winter, while our AC unit's or whatever we had sticking out the window isn't available, and the common recomendation is to put it on a cookie sheet, or perhaps on top of a metal filing cabinet, or other flat surface.
Would the antenna performance be better if I used either a cone, or at worst a large mixing bowl set up side down, to simulate that 45 degree downward slope on the radials?
Alternatively if I just used a couple of iron straps (for the magnetic mounting) and bent them down and perhaps made a stand out of them, would that give better performance?
I may be able to try some of these this winter, though I don't have a SWR meter available (yet) to get good readings. Mostly I would be impressed if I got better response from some of the repeaters in the area off of a hand held.
Any one else with ideas?
-Rusty - kc0vcu
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RE: groud planes for mag mounts...
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by KE4DRN on October 11, 2005
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hi rusty,
As long as you use steel for the sheet and the bowl is large enough, give it a try !
I have half of the metal cage from a 20" floor fan that I put the magmount on and it works great. I did use it with a wattmeter to check my swr.
Is this for 2m ?
73 james
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RE: groud planes for mag mounts...
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by N3ZKP on October 12, 2005
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Rusty,
With an mag mount, I doubt that you notice any practical difference between a cookie sheet and a mixing bowl as long as the surface area is about 24"x24". From my experience, a bowl with that kind of surface area is REALLY BIG and REALLY EXPENSIVE.
If you have something in the kitchen already give it a try; I wouldn't spend any money on the project.
Lon
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RE: groud planes for mag mounts...
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by N3ZKP on October 12, 2005
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Rusty,
With an mag mount, I doubt that you notice any practical difference between a cookie sheet and a mixing bowl as long as the surface area is about 24"x24". From my experience, a bowl with that kind of surface area is REALLY BIG and REALLY EXPENSIVE.
If you have something in the kitchen already give it a try; I wouldn't spend any money on the project.
Lon
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RE: groud planes for mag mounts...
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by WB6BYU on October 12, 2005
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You can solder 4 radial wires to a tin can lid and attach
it to the mag mount. Then you can position the radials
at whatever angle you wish.
In the practical world, I doubt you will notice much
difference in performance. The theoretical impedance of
a quarter wave radiator on a flat ground plane is 35 ohms,
and over a single vertical radial (making a dipole) is
72 ohms. Either will give a low enough SWR to keep your
radio happy.
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RE: groud planes for mag mounts...
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by WA4PTZ on October 18, 2005
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I like the tin can lid idea, it has the best chance
of helping. And yes, changing the angle of the radials
relative to the antenna can often improve the SWR
and the signal. Experimenting is what we are about.
73 ,
Tim
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RE: groud planes for mag mounts...
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by KC0VCU on October 18, 2005
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I am becoming somewhat fond of the tin can lid idea myself. I have some reasons of my own to want to get my antenna outsied, yet I still want a fairly low profile or reduced visibility/impact setup. I live in an appartment, with those nice restrictions that say you can't put anything on the outside of the appartment. (No deck, so I can't mount something there either.)
The mag mount sitting on the air conditioner was fine during the summer, but I suspect they would notice a 2 ft. by 2 ft. piece of metal sitting out there with a few inches of snow on it.
My main reason for wanting to get an antena outside does not have to do with rf exposure, at the power levels I am dealing with that is not generally an issue. No, I am somewhat more concerned with the fact that the building is a brick structure, with a wood frame interior. From what little I know of 1960's construction I strongly suspect that there is a nice wire mesh between the wood frame and the brick facing to give the masons something a bit more comfortable to mortar the bricks against. I have no doubt that this is acting as a very nice Faraday cage. Mostly because of the trouble I have had just picking up regular FM and TV signals in both UHF and VHF bands. So even a thin wire dipole tacked to the side of a window frame is likely to give me better reception than any antenna inside of the building.
Obviously some spacing from the structure would be better, though again we have to keep it at a level where I have not mounted anything permanently to the building. No bolts, wood screws, driling and so on. Yes getting a co-axial cable through the wall will be interesting, though I think I can get it done.
Yes the antena at this time is a dual band 2m/70cm one, Actually with a very small magnet under it.
I presume that the ideal length for the radials is electrically the same length as the existing antenna. So if the antena has a loose coil at the base, I should consider that as a lengthener for the height? Or should I consider that a trap for the 70 cm band? Length of the antena from magnetic mount to the tip is 17", the coil is two turns half an inch diameter, and about half an inch from begining of first turn to the end of the second turn.
Thanks everyone for the feedback so far.
73
-Rusty - kc0vcu
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RE: groud planes for mag mounts...
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by WB6BYU on October 19, 2005
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The radials should be about a quarter wavelength long.
About 18" or 19" should work close enough - you will have
to experiment to find what angle gives best results on
440 where the radiation from the radials will have lobes
and nulls. Some of the lobes will be out of phase with
the radiation from the radiator, which will cause the
signal to cancel. And you should be able to get by with
just two radials.
The best adjustment tool you have is probably your S-meter.
Adjust the antenna for best received signal strength from
a somewhat-distant repeater. If you are starting with a
reasonable design and the cable isn't too long, the SWR
by itself shouldn't make much difference.
If you have problem getting the antenna outside, consider
mounting it on the inside of a window. You can either
stick it to the inside of the window, or hang it on the
back of the curtain where it is out of sight. For a
mag mount, put it on the window sill and run the radials
along it. (If the window has an aluminum frame, you
might try using that as your ground rather than, or in
addition to, the wire radials.)
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RE: groud planes for mag mounts...
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by N3TTN on December 18, 2005
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My understanding is that with mag mounts, which are by default designed for automotive use, the bigger the metal surface area, the better. That's why a car body works very well, because there is lot's of metal surface area. You might try a good sized sheet of galvanized steel, that should be fairly cheap to buy and will give you lot's of metal surface area to mount your antenna to, and you can put down as much as your patio or balcony will allow for a better ground plane. If you want a 1/4 wave ground plane with sloping radials, just make one, they are really easy to make and work quite well. The ARRL antenna handbook has designs for 2 meter, 220, and 440 ground planes that are very easy to make.
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