2m/70cm mag mount, RG6 - ground plane?
I've got a 37" mag mount that is sold as a dual band antenna for 2m/70cm.
It comes with 12 feet of RG58 and a PL259.
I don't know the impedance of the antenna, but the best swr I can get by adjusting the whip is about 1.5/1 on both bands.
I can't install a permanent antenna. I'm on the 2nd floor inside a 1920' era brick building surrounded by other brick buildings and urban infrastructure.
I want to get a temporary antenna on the roof for use with local repeaters.
There are a bunch of 6" iron vent pipes on the roof.
I have a boat load of RG6 that I got for free.
I'd like to feed this mag mount with RG6, and run the 75' I need from the antenna down to the shack.
I'm not sure if I will need to match this somehow.
I'm even less sure if I need a groundplane, or even just a "tiger tail"
Since i believe this would qualify as an "end fed" dipole, I think not - but I don't know.
Thanks in advance
It wouldn't be an "end fed dipole" because it's not 1/2-wavelength long.
Mag mounts aren't great. Can you "stick" it on a rooftop air conditioning unit or something having a substantial amount of horizontal metal for a ground plane? An iron pipe probably isn't very effective, although better than nothing.
RG6 is 75 Ohms. If you use it, your SWR can be anything from 1.0 to 2.25:1, all of which isn't bad. I'd have no problem using it; the challenge is the connectors, since RG6 is made for crimp connectors only. You can use Type F crimps and then F-to-UHF adapters, which at 144 MHz work pretty well and at 440 MHz might have a bit of loss.
(RG6 is quite low loss, and so are F connectors; any real loss could be in the adapters, but it shouldn't be much.)
75' rg6 144 mhz about 2.5 db line loss
75' rg6 440 mhz about 4.6 db line loss
does not include swr loss.
It's far better than RG58/U and a tad better than RG8X. Not bad, for free.
Originally Posted by KC9QVL
True free is always good
Maybe a diy gain antenna on the roof. J-pole etc
Thanks all - yeah, I figured on the losses. I have no problem with the UHF repeaters around here. There is a 2 meter machine in Queens that I have a hard time hitting from my shack. I can get it near DFQ from my bedroom with 5 watts... from my shack it is a challenge with 50 watts. If you want a lesson in attenuation, this is it!!!
I built a dual band quad that hangs from the ceiling in my shack, and it almost solves the problem for the HT. I can get in on 5 watts, but noisy.
We are also getting ready to show the apartment, so the quad is going to have to go....
I did think about a jpole, even a twinlead () jpole mounted inside pvc pipe - but that is already too much to consider mounting up there. It has to be really inconspicuous, easy to take down, and low value if somebody should snag it...
I actually thought about using the coax itself as some kind of bazooka or even "fan" type of deal with a quarter wave radiator for each band.
It seems weird, but if I could show you a blueprint of the building and the layout of the neighborhood and terrain you'd understand immediately...
Yes, the RG6 was free and I have about 150 feet. There's more where that came from, so I'd like to take advantage of it... Good quality stuff too...
I forgot to add, the RG6 blends in with all the Cable TV runs down the side of the building... (no I won't run it adjacent to those cable tv runs)
If I could figure out how to run the coax from the shack to the bedroom inconspicuously I could actually stick an antenna outside the bedroom window... You'd have to see the apartment, plus getting ready to show it I can't add to the clutter. This might still turn out to be a better option. Even if I have to suck up the losses in a run of RG58. (to the bedroom)
Connectors - I heard of some kind of paste/flux you can special order for soldering to the foil? otherwise, I'd crimp and use adapters...
Last edited by AE2CS; 09-14-2010 at 01:42 AM.
another idea - this is where you can say I'm really crazy.
I have an old set of rabbit ears, that have a second set of radiators attached to the primary "ears" for UHF.
It would be easy to tune this for dual band operation and there are screw terminals for attaching the coax. (I could waterproof it).
If I put that up there, and just used some black duct tape to hold it in place at the lip of the vent pipe, in a vertical "V" or an "L"
Not really any gain, but it would be inconspicuous, and disposable...
Do whatever works!
I worked Hawaii on two meters from a hotel room -- seriously.
Of course that takes a duct we call the "Pineapple Express," which is not an every day occurrence; in fact it happens maybe a couple of times a year, and you just have to be there to catch it. Still, it's a 2600 mile contact and signals were good enough to work.
My favorite hotel for this stuff is one in Newport Beach, on a hill overlooking the ocean. Hams have gone there for years to work the duct.
Timing is everything.
From where I live (at home) I can work 2m FM simplex 150 miles in several directions, just using a homebrew collinear antenna on the roof. Working the repeaters in Santa Barbara (80 miles northwest) or San Diego (120 miles southeast) is very, very easy, and they're very strong.
With an indoor antenna, I can't hear any of them.
Do you have a model number for the antenna?
It is almost - but not quite - the length of my dual-band mobile antennas, which are "no ground plane required" models. I haven't really tried with the others, but I have a Diamond NR-770HA that I bought over 15 years ago. It went to college (dorms) and all my apartments, worked fine on a mag-mount without any sort of ground plane or tails. At the dorm, I set it on a concrete ledge outside the window. In the apartments I could only set in on the windowsill.
But those antennas are all about 40" tall without the mount, so yours may be different. Not that that means it wouldn't work - I've used all sorts of magmounts in less-than-ideal locations stuck on whatever's handy. Radiation pattern might not be the greatest, but they'll still function.