Plastic pipe beside j-pole

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Johnson1522

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I have a copper J pole set for 2 meters and it works great. My radio is in a basement, but the run is just under 50 feet of lmr400. I often have a scanner running, but the reception is (as expected) horrible in the basement. I want to run a strand of rg59 up and mount a rubber ducky outside on the same mast as the ham antenna. The idea is to mount the rubber ducky on a short length of pvc that will sit beside (but not in contact with) the non-radiating portion of the jpole. Can anybody suggest why I might not want to do this? I can't imagine I would use both radios at the same time.
 

jaspence

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Depending on the output of radio connected to the j-pole, it can overload the scanner front end if the antennas are too close together. This can damage the scanner. I would separate them by at least one wave length of the transmitted signal and more if possible. Even if you don't plan to use them at the same time, forgetting to turn off the scanner would be all it would take. In my area, I often have the scanner and 2 meter running together when storm spotting, but I do not have any problem with using the rubber duck on the scanner so no worries on overload.
 

jonwienke

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Even if you don't plan to use them at the same time, forgetting to turn off the scanner would be all it would take.
Whether the scanner is turned on or not is irrelevant. If you feed too much RF into the antenna input, you will fry it.

Your best bet is to vertically separate the antennas, as well as offset the scanner antenna horizontally from the mast to minimize interactions between the scanner antenna and mast. My setup has the scanner antenna (an ST-2) 10 feet below the main antenna and offset 4 feet horizontally from the main mast.
 

mmckenna

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Worse case:

If the antennas are close enough together, you can induce enough RF energy off the J-Pole transmitting antenna to get into the scanner antenna and down to the scanner. Turning the power off on the scanner does NOT disconnect the antenna from the soft/expensive bits inside the radio. Transmitting with your 2 meter radio, even with the scanner turned off, can still damage your scanner.
It might be rare, but it does happen, especially with consumer grade scanners.

Less than worse case:
Keying up your transceiver will cause enough RF energy to get into the front end of your scanner to disrupt reception. This is called receiver desene. While it may not damage anything, it'll keep you from receiving anything on your scanner while you are transmitting with the other radio.

Either way, you want a lot of separation between the two antennas.

If you do something like this, you ideally want quite a bit of vertical separation between the antennas if you are going to try to do this on the same mast. Either that, or quite a bit of horizontal separation, as in several feet at least.

You'd also do better if you used a higher grade cable than RG-59. That's pretty low end stuff and you'll lose some of your received signal just in cable losses before it can make it to your scanner. Use RG-6, specifically the RG-6 QS (Quad Shield) stuff if you are going to have both antennas on the same mast. It's also got less loss, so more signal will make it to your scanner.
 

Johnson1522

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Thanks for the replies, everyone.

@jonwienke I could certainly do the horizontal spacing, but vertical space is at a premium so it wouldn't be possible. It may just be worth it to throw up cheap pole to act as a different mast.

@mmckena "Turning the power off on the scanner does NOT disconnect the antenna from the soft/expensive bits inside the radio." Turning off the scanner does disconnect antenna feed for this setup. Thanks for pointing that out, though.

As for the cable, I didn't realize that the loss would be great enough over such a short run to warrant any extra money for the cable. I'll certainly take your suggestion to heart.
 

WA8ZTZ

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Thanks for the replies, everyone.

As for the cable, I didn't realize that the loss would be great enough over such a short run to warrant any extra money for the cable.
Using RG-6 cable will not result in extra expense... it is cheap and readily available. You will need to obtain appropriate adapters to go from the "F" connector on the RG-6 to whatever is on the scanner and antenna (BNC?).

Separate the scanner and 2 meter antenna as much as possible.
 

popnokick

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The rubber ducky attenuator.... er, "antenna" is a pretty horrible choice for an external antenna. A home-made quarter wave ground plane would be a much better performer than a rubber attenuator. Also look into this Off-Center Fed scanner antenna (mounted away from the J-pole) which actually offers gain over a dipole (and certainly a rubber attenuator)... and covers multiple frequency ranges in a single antenna -
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
 

Johnson1522

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@WA8ZTZ I already have the RG59, but I see your point. Thanks!

@popnokick I really like this idea. I think I'll scrap my original plans and do something like what you've mentioned. Thanks for the suggestion.
 

byndhlptom

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Plastic pipe beside J pole

The fact that your intended mast is plastic becomes somewhat immaterial if you are running a piece of coax either through it or taped to it.

A J pole is sensitive to adjacent metal. whether it is a mast, piece of pipe or a coax cable, they all become a metal "rod" in the J-pole's radiation "zone". You may find that your J pole is not working as well as it once was

I agree with previous poster that a basic 1/4 wave ground plane would be a better external antenna than a rubber ducky. They are easy to make and there are plans for multi band versions is you look...

$.02
 

Johnson1522

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Thanks for the feedback, byndhlptom. I just can't figure out quite how to size a 1/4 wave ground plane for the frequency range. Some local stations are in the 140-150 range, a few others are 400ish, but a lot of other traffic has moved to he 850ish range.
 

WA8ZTZ

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Thanks for the feedback, byndhlptom. I just can't figure out quite how to size a 1/4 wave ground plane for the frequency range. Some local stations are in the 140-150 range, a few others are 400ish, but a lot of other traffic has moved to he 850ish range.
For receiving only, don't worry too much about it. Have got a 1/4 wave ground plane here cut for 2 meters ham (about 19") and it works fine on aircraft band (118-136) and vhf high band (150-174) as well. In fact, it also works OK on the UHF frequencies.

However, if you want an antenna specifically designed for wideband reception, check out a discone antenna.
 

byndhlptom

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