Roll-up "slim jim" J-Pole question

I think I know the answer to my question, but I want to be sure, seeing that antenna theory isn't my specialty.

A while back, I bought a KG-UV6D v2 from a seller at the QRZ swap meet. Included with the radio was a roll-up 'slim jim' j-pole antenna. Threw it in a drawer since I was going to use the HT outdoors while walking or cycling. However, now that the weather has turned colder, I won't be doing those outdoor activities quite as often, but I still want to get some use out of the HT, so I want to use indoors sometimes instead of my mobiles being used as base radios. So with that thought in mind, I dug through the drawer and pulled out the roll-up antenna.

I want to put it along the inside frame of my bedroom window. I'm thinking it should be hung vertically, seeing that all my other antennas are vertical, but I just want to be sure. So, that's my question; do I hang it vertically?

Thanks in advance for any information provided.

Vertically sounds correct.

Do you have a picture or a model/manufacturer for this roll up J-Pole? I'd like to see it and maybe some reference dimensions of it or maybe a picture of it next to a yardstick or something.

NWI_Scanner_Guy said:

I think I know the answer to my question, but I want to be sure, seeing that antenna theory isn't my specialty.

A while back, I bought a KG-UV6D v2 from a seller at the QRZ swap meet. Included with the radio was a roll-up 'slim jim' j-pole antenna. Threw it in a drawer since I was going to use the HT outdoors while walking or cycling. However, now that the weather has turned colder, I won't be doing those outdoor activities quite as often, but I still want to get some use out of the HT, so I want to use indoors sometimes instead of my mobiles being used as base radios. So with that thought in mind, I dug through the drawer and pulled out the roll-up antenna.

I want to put it along the inside frame of my bedroom window. I'm thinking it should be hung vertically, seeing that all my other antennas are vertical, but I just want to be sure. So, that's my question; do I hang it vertically?

Thanks in advance for any information provided.

Click to expand...

A J-pole antenna is an end-fed half-wave antenna. The bottom portion of a J-pole, the part where there are two antenna elements side-by-side, is a transformer which is used to match your feedline (coax) to the feed point of the antenna. This transformer is necessary because the impedance of an end-fed half-wave antenna is not the 50 ohms that your transmitter is looking for.

If the frame of the window is metal, especially if it's steel, the magnetic field generated by the transformer section of the antenna will be disturbed by the window frame if the antenna is too close to the window. The result is that the antenna may not work very well and may not present a good match to the transmitter and feedline.

My suggestion would be to find a clear section of outside-facing wall where there aren't likely to be any electrical cables in the wall. Hang your J-pole on the inside of the wall. Yes, the drywall, wood, and siding of the wall may attenuate your signal somewhat, but that will be better than transmitting into a poor match.

The first 10 years I was a ham, I didn't have any outside antennas. Just J-poles hanging from the walls. That was sufficient for getting into local repeaters.

That KG-UV6D v2 looks like a nice radio. I wish I had something that good when I started out.

The other posters provided you with all the information you need. But your posting brought back some memories. When I first started in ham radio (1965) all I had was a Heathkit Twoer, that I just assembled, and various scraps of wire. So I built a folded dipole out of some 300 Ohm twin lead and attached a short section of RG-58 coax. I then taped it up on the wall about 6 feet over my head. The wall was an interior wall that connected two bedrooms. It worked great and I operated like that until I could afford a 3-element beam and more coax. I still have the Twoer and the 3-element beam. I don't use the Twoer any more, but the 3-element beam still works good.

Thanks for the memories.

I guess I've never pictured a J-Pole of a roll up design in my head.
I'm used to making them out of copper pipe!

W9BU is correct about metal framed windows messing with the SWR or impedance matching.
I had a nice and tall window but not very wide that was on the side in the direction I was needing signal from.
The only problem was the metal screen. I replaced the screen with the nylon mesh type and it ended up working pretty darn well for an indoor antenna.

I also had a huge yagi for the 900 MHz band. I hung it from a ceiling but it did not work worth a darn. Then I realized the exterior of the apartment is Stucco and the Stucco was applied to a steel mesh. The steel mesh ended up blocking most of the signal I was after.
I moved that yagi so it was aiming through the same window that I'd had the J-pole in and it also worked fairly well although I could not get the exact aim I wanted. I only had a few degrees of aiming change before the steel window frame or stucco steel mesh messed things up but it still worked better than anything else until I was able to go on the roof with a rotor. It sure looked ugly though!

I also played with RX only HF loops indoors. They worked great as I was on the top floor with nothing above me but a drywall ceiling on wooden studs and then a rubber membrane flat roof applied to a fiberglass type insulation that passed those signals through very well.

It's all mounted above the flat roof today though.

Here's a link to the MFJ Roll-up J-pole: MFJ Enterprises Inc.
I used one several years ago while in FL to work STS in orbit. Threw it up in a tree vertically and got a lucky shuttle pass. But yes.... keep it away from metal objects that could distort/mismatch the performance.

If constructed and tuned properly, a roll-up J-pole can work surprisingly well. Yes, you want it to be vertical (which is why many of them have a hanging loop on the top), and definitely keep it away from metal/conductive objects. And like any other antenna, get it up as high as possible.

Thanks for all the replies. They are appreciated.

Here is a link for some very nice J pole dipoles made from 300 ohm or 450 ohm depending on the model. All cost about $25.00 give or take depending on which model.
They work great and the guy who makes them will cut them for a specific frequency if desired.
N9TAX.com
I used a duel band 2M/70 CM as a second antenna while net control for the "Bike Greater Yellowstone" seven day bicycle race and it worked as well as my vertical.
I also use one of his antennas for GMRS and it works quite well also.
Don't know the guy, but got the antennas quickly and no extra charge to cut them for any specific frequency.
73's John