Indoor antenna for gmrs

[Josh380]

I'm thinking about buying this: http://pages.ebay.com/link/?nav=item.view&id=321557457154&alt=web

What do you guys think? The design is perfect for my needs.

 

[mmckenna]

1/4 wave antenna.
The issue won't be your antenna, it's going to be the wiring, plumbing, foil backed insulation, vapor barriers, etc. in the building that are going to cause you the issues.

UHF tends to work via line of site. It does penetrate buildings pretty well, but it's going to be severely attenuated.

Would be suitable for short range use, but I wouldn't expect much. At $18 you wouldn't be losing much.

 

[Josh380]

I have a room separate from the house. It has foam insulation, and minimal wiring (4 AC outlets, a light switch, and a light). No plumbing to worry about. The room has a wood frame and wood siding, and shingles, not tiles, on the roof. Not to mention I live on a hill that peaks over much of the area.

I'll be using my (new to me) Kenwood tk-840n. I have been using my ht1000 from my room, and I have no problem getting into the repeaters in Los Angeles (I live about an hour from LA).

I guess maybe I should've provided that info first post.

 

[cmdrwill]

You should also look up the RF Exposure limits. You do not want a transmitter's antenna anywhere near you. Max was 4 watts,

 

[Josh380]

Hadn't thought of that. I emailed them and asked. Not sure how helpful they'll be considering it's China.

Or I completely misunderstood what you said and maybe you meant this: http://www.rfcafe.com/references/electrical/fcc-maximum-permissible-exposure.htm

And if I'm reading it correctly, it's 3W at 6 min. Maybe not such a good choice after all.

 

[mmckenna]

Yeah, I would not be wanting to run a mobile radio at high power with an antenna like that in the same room as me.

If your shack is free of any shielding, then maybe something like a ground plane antenna in the attic would work. Personally I'd really want an external antenna. Getting it up higher would really improve performance. Even if you make your own 1/4 wave ground plane (easy), you'd likely do better.

Either that, or keep the power output of that Kenwood down at low power.

 

[Josh380]

Well, there isn't an attic either. Put simply, a large shed converted to a room.

I can't really mount an antenna outside, and I don't want to give myself cancer either. What about the Slim Jim antenna? My options are limited here and I don't have a vehicle to put this in right now. Gotta find a way to run it.

 

[mmckenna]

You want the radiating element well away from you. Where you put that depends on the structure.

A Slim Jim is just a J-pole. They work pretty well, but they are not going to negate the RF radiation issues.

 

[Josh380]

So. Back to square 1. I'm out of ideas....unless I can find a way to hide an antenna.

 

[mmckenna]

Flag pole.
Few people will complain about a flag pole. Get/make one made out of a non-conductive material. Maybe heavy wall PVC pipe, or a steel pipe with the last couple of feet made out of PVC. Hide a base antenna or even that J-Pole inside. Route the coax cable up the inside.

We've got a cell phone carrier that did that here in town. Few people know what it is.
I've got a co-worker that lives in a trailer park. Antennas are not allowed. So, to solve the issue he installed a flag pole with an insulated base. He uses it for amateur radio HF work. Talks around the world and no one knows he's got an antenna hidden in plain sight.

You can also hide an antenna in a tree.

Disguise the antenna as a vent pipe on the roof. Take a look at the VentTenna. Untitled

A simple mobile antenna properly installed on a piece of sheet metal tacked to the shed roof will blend in really well.

 

[mformby]

I'm thinking about buying this: http://pages.ebay.com/link/?nav=item.view&id=321557457154&alt=web

What do you guys think? The design is perfect for my needs.

I have to question the 3db gain rating. By design that would be hard to do. Probably closer to a unity gain. I would add a metal plate on top of it to act as a ground plane. 36" square should be sufficient.

 

[Josh380]

So I guess I'm gonna have to go with one of those 1/4 wave pre-tuned nmo mount antennas and get a mag mount for it. Small enough for me to pop on the roof and not be an eyesore. I imagine my rf exposure should be limited as long as it's above me, right?

 

[N4KVE]

I use this in my porch on the 2nd floor. Works great. Check it out.
UHF 440-470 MHz Slim Jim Antenna | KB9VBR J-Pole Antennas

 

[Josh380]

My roommate would hang me from that thing if I tried to put it up.

 

[mmckenna]

I have to question the 3db gain rating. By design that would be hard to do. Probably closer to a unity gain. I would add a metal plate on top of it to act as a ground plane. 36" square should be sufficient.

Well, dB is usually referenced to a known point. dBd, dBi, etc. Since the claim is only dB with no qualifier, the 3dB claim means nothing.
Basically all it's saying is it has twice the gain of an unknown reference. That reference could be anything, including a 50Ω dummy load.

 

[mmckenna]

So I guess I'm gonna have to go with one of those 1/4 wave pre-tuned nmo mount antennas and get a mag mount for it. Small enough for me to pop on the roof and not be an eyesore. I imagine my rf exposure should be limited as long as it's above me, right?

Yeah, that would work, as long as it has a ground plane under it.
If the shed roof is metal, then you're good to go. The metal shed should shield you.
If the shed roof is not metal, then you'll need to provide a ground plane. A piece of 12" x 12" sheet metal would work fine for UHF frequencies. Tack it down on the roof and drop the mag mount in the middle of it.

 

[N4KVE]

My roommate would hang me from that thing if I tried to put it up.

It's barely 2 feet tall.

 

[KD8DVR]

You want the radiating element well away from you. Where you put that depends on the structure.

A Slim Jim is just a J-pole. They work pretty well, but they are not going to negate the RF radiation issues.

A slim jim is actually an entirely different design. Similar, but different.

http://www.jpole-antenna.com/2012/10/21/slim-jim-vs-j-pole-antennas/

 

[Josh380]

I have another thought..would a metal fence (like the ones you see surrounding a pool, not chain link) be a suitable ground plane? If so, I've got one less than 2 feet from the room. It wouldn't be super high, but over my head, and I'd have a pretty short coax run. I could use a mirror mount, mount it to one of the vertical bars, and pop the little 1/4 wave antenna on there.

And btw there's no metal roof. Think tuff shed, but bigger. No excessive metal anywhere. There is a strip of metal that runs just below the shingles, that's about it.

 

[mmckenna]

It'll work, but ultimately it'll screw up the radiation pattern. To get full omni-directional coverage, you need a ground plane all the way around the antenna. I've seen antennas with as few as 3 radials at 120˚ increments. 2 (top rail in each direction, probably isn't going to work as well.

I'd do the 12" diameter metal sheet on top of the top rail and put the mag mount on that. Having a good ground plane under the antenna is going to help it work well in all directions.