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Author Topic: Use J-pole antenna in second story apartment window  (Read 154 times)

Posts: 2

« on: August 15, 2019, 08:33:24 PM »

What concerns should I have about placing a J-pole for 2-meters in my apartment window?  I know that there will be RF but is there a minimum no. of feet between radio and antenna?  I have a max 50 W TYT TH-9800 mobile I want to use inside.  I'm a new ham and trying to learn about this hobby. 

Posts: 2784

« Reply #1 on: August 15, 2019, 08:50:38 PM »

You don't say anything about what kind of operating you plan to do on 2M.

You might consider the relative advantage of a directional antenna (dipole or yagi) or
a balanced antenna (dipole or yagi). Neither of which the J pole offers. Will you want
vertical or horizontal polarization?

Here's some advice for a very simple antenna to make:

Posts: 1378

« Reply #2 on: August 16, 2019, 02:53:46 AM »

A 2m J-Pole is very easy to make from 300 Ohm ribbon, you need to coil the feeder around a small plastic tube to form a feedline choke to avoid the feeder becoming part of the antenna. A search on the web will find plenty of details for ribbon cable J-Poles. Equally, you could make a dipole, again use a feedline choke.

You might want to run less power than 50W with an antenna so close, the potential for interference is quite significant when in close proximity to your neighbours.

73 Dave

Posts: 3213

« Reply #3 on: August 16, 2019, 05:16:46 AM »

Placing the antenna near the window should not be a problem. There are many reports of people successfully taping wire antennas to the glass. This might change the SWR or other antenna parameters based on the surrounding wall materials, the window frame, the type of glass, etc. but all you can do is give it a try.

Using a 3 element directional beam, for example, would ensure that more of your RF power is directed out the window rather than into the room (this is called directional gain). It is also a much shorter antenna (39 inches vertically) than a J pole but it will require more depth (about 43 inches) into the room and it cannot be easily fastened to the window.

If you are concerned about RF exposure, the ARRL has a nice resource page to help you assess your situation:

There is an FCC requirement that you conduct an RF exposure assessment if you run more than 50 watts of power on 2 meters (70 watts on 70 cm) into your antenna feedpoint. If you exceed these levels, there is a calculator for that purpose linked near the bottom of that page.

- Glenn W9IQ
« Last Edit: August 16, 2019, 05:30:28 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.

Posts: 2

« Reply #4 on: August 16, 2019, 08:36:53 AM »

I appreciate the specific information offered.  I want to learn about efficient and safe operation.  Yes, I studied the book and got my General, but there is nothing like real-world experience to help out a new ham.  Thanks.

Posts: 184

« Reply #5 on: August 16, 2019, 08:42:12 AM »

I have been known to hang a Slim Jim from a second story balcony inside my house. (for testing/backup) Best to keep it way from any metal.

I also use it in the field by hanging it from a tree. I have also deployed it at hotels while traveling.

When ordering this antenna you can request whatever type connector you prefer.

I use this antenna as well and it is very small and unobtrusive, and it performs remarkably.

Posts: 1446

« Reply #6 on: August 16, 2019, 09:44:05 AM »

I appreciate the specific information offered.  I want to learn about efficient and safe operation.  Yes, I studied the book and got my General, but there is nothing like real-world experience to help out a new ham.  Thanks.

The real-world experience comes from your own testing/experiments.

Every situation is different, when I was using an indoor antenna, thought it would work best by the window, that location resulted in the worst receive, and performance. Most antennas don't like to be close to other objects.

Results were better, when the antenna was 6 feet away from any objects.

Regardless of any RF exposure tests. I personally wouldn't sit in a room, with an antenna that's putting out 50 watts.
My indoor limit is 10 watts, save 50 watts for a proper antenna installation, or mobile operation.

Posts: 18386

« Reply #7 on: August 17, 2019, 10:02:55 AM »

I've had good luck with a J-pole pinned to the back of a curtain over a picture window, but smaller
windows (especially those with aluminum frames) don't always work as well.  I've also used a suction
cup to hang ground planes for 2m or 440 on a window - that was my primary 440 antenna for several
years, and it actually worked quite well:  I had less than 3 feet of coax on it.   You can also build an
antenna on the inside of the window using copper foil:  here are some ideas.

However, some tinted glass contains a metallic coating that can block RF.

You really do have to experiment with indoor antennas to find the best location:  you are dealing not
only with the direct path to the transmitter, but also reflected paths off appliances, ducting, etc.  Often
a window facing the proper direction is a good place to start, but you might find somewhere else that
works better.  I knew one ham family that had a piece of tape on the living room carpet:  that was the
place to stand to hit the repeater with an HT.

While I don't think 50 watts is dangerous for casual use, I try to avoid the high power settings on my
equipment unless absolutely necessary:  I'd rather improve the antenna somehow so I can cover the
path at lower power, at least for the more local repeaters / those I use most.  Generally 5 or 10 watts
is adequate with a J-pole or ground plane antenna if the repeaters are designed to cover your area.

Posts: 7768

« Reply #8 on: August 17, 2019, 12:56:36 PM » there a minimum no. of feet between radio and antenna?  I have a max 50 W TYT TH-9800 mobile I want to use inside.  

The ARRL RF Exposure Calculator at the link below says 4.4' for a controlled environment and 10' for uncontrolled. The parameters used are:
2.4 dBi
148 MHz

Controlled environment applies to you because you are aware that there's a nearby RF source. Uncontrolled applies to anyone who isn't aware, such as people in the adjacent apartment. For a controlled environment you may average the RF power over a 6 minute period. For uncontrolled you may average over a 30 minute period. For example, if you transmit for 1 minute, receive for 1 minute, and continue like that your average power is 25 watts.

« Last Edit: August 17, 2019, 01:07:38 PM by KH6AQ » Logged
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