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Author Topic: J pole installation question  (Read 2859 times)
KF5PGT
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Posts: 38




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« on: April 14, 2012, 10:41:01 AM »

I just got my tech license and a 2 meter rig. I was thinking of throwing a J pole in my attic temporarily while I study for my general. I'm planning on putting a bigger multi-band antenna in my yard once I upgrade, so I don't wanna commit to a 2 meter antenna in my yard just yet.

If I was to hang a 2 meter J pole antenna in my attic, could I just attach it directly to a wooden beam? Is that safe?
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K7KBN
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Posts: 2754




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« Reply #1 on: April 14, 2012, 10:46:28 AM »

 Probably wouldn't be a problem, but I wouldn't have the antenna itself in contact with the attic wood (it IS wood, right?).  A couple of inches of clearance would be safer and wouldn't tend to detune things.

Experiment!
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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Posts: 12983




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« Reply #2 on: April 14, 2012, 11:00:00 AM »

The common copper-pipe J-pole should be supported below the matching stub
(the portion on the bottom where there are two parallel pipes.)  If you have
a short stub of pipe sticking down from that point you can mount that directly
to a wood beam.

Attaching anything higher up on on the antenna may detune it somewhat, though
I've gotten by hanging it from the top using plastic rope (usually baling twine.)
Tie the rope around the copper a few inches down from the top and hang it
from a nail with the top of the antenna 6" or so below the ceiling.
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KF5PGT
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Posts: 38




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« Reply #3 on: April 14, 2012, 02:18:42 PM »

I think I came up with a solution.

I took apart a microphone stand and used the base for the J pole. It works and it's free standing with about 8-10'' of clearance. I was mainly worried about any fire hazard present if the antenna was directly touching and wooden rafter. It's a temporary solution, until I get a HF rig and my General ticket.

Now, if I could just get someone to respond to my CQ!

Thanks for the info!
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G8HQP
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Posts: 119




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« Reply #4 on: April 14, 2012, 03:48:26 PM »

The J-pole is essentially a narrow-band end-fed half-wave dipole, with a matching section. It is easily detuned by nearby objects, so not ideal for indoor use. A folded dipole with a suitable balun may work much better in a loft.
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KF5PGT
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Posts: 38




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« Reply #5 on: April 14, 2012, 04:11:26 PM »

Then what might be a better option that I can homebrew? Dipole? Ground plane?
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K7KBN
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Posts: 2754




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« Reply #6 on: April 14, 2012, 04:21:12 PM »

Can you access repeaters with the j-pole in its current location?  I can't remember the last time I heard a CQ on 2 meters.
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73
Pat K7KBN
CWO4 USNR Ret.
KF5PGT
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Posts: 38




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« Reply #7 on: April 14, 2012, 05:57:20 PM »

I think I can key up a local repeater, but no one ever responds. So I'm not positive.
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W6CD
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« Reply #8 on: April 14, 2012, 06:15:01 PM »

Typically one does not call CQ on 2 meters, rather just say your call sign and then sat 'listening'.  Ask for a radio check, new antenna check, say you are new ham, etc.  If nobody comes back there is nobody there (doubtful) or something is wrong with your setup. 

Suggest to contact a local club or the repeater onwer (probably one in the same) - someone will help a new ham.  And you will make some new ham friends, a best part of ham radio.
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KF5PGT
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Posts: 38




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« Reply #9 on: April 14, 2012, 06:46:29 PM »

I haven't actually called "CQ", I just gave my callsign. No one has responded.

I'm think I'm getting to the repeater because when I transmit, it'll give me a beep and it doesn't do that when I'm working other frequencies. Sometimes, right after I transmit the repeater will give its identification announcement.

I think I'll take your advice and get in touch with someone from that club. I'm really jut trying to get someone to respond so I know my rig and antenna are functional.  I can recieve just fine, but I'm starting to wonder about transmitting...
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N6JSX
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Posts: 216




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« Reply #10 on: April 15, 2012, 08:04:24 AM »


"If I was to hang a 2 meter J pole antenna in my attic, could I just attach it directly to a wooden beam? Is that safe?"

The only real issue will be near-field reflections that can be caused my most anything even PVC/wood close to the radiating elements. These near-field reflections can de-tune the J. {I made a TV twin lead J and put it inside PVC to encapsulate making it water-proof. This was used as a sneaky hidden T/Fox hunt antenna that I tossed in a flowing 18-24" deep creek. I found the TV J was 1:1 until I put it inside the PVC, it shot up to 2.8:1 due to near field reflections. Shocked  I re-tuned the J taps and got it back to 1:1 inside 3/4" PVC. Oh the hunters really hated me fro this one as no one wants to get wet - it did radiate well from under water! This hide is still in Southern CA T-Hunt folklore!}

As far as being safe, there should be no issue providing you are mounting it as far from metal rafter/roof plates as possible. However even the close proximity of wood/roof may de-tune your J, requiring you to re-tune in place.  

If I'd be making a J for attic use I'd make it from 3/8" or 1/2" copper pipe (as there is no wind loading to worry about). You could drill one hole thru the pipe top to vertical mount using a drywall screw into top roof rafter. Try to place the J as far from all near parallel wood as possible - remember you already have a line-of-site obstruction, aka 'roof', that will cause you limitations but your added antenna height will surpass any rubber-ducki helical dummy load!

I have made TV-J's and hung them from curtain rods behind drapes, set a copper pipe  in a room corner, and sport a copper pipe  on top my tower. They are very forgiving, cheap/easy to make and have a very low angle of radiation giving the most flat land distance possible. Experiment, build, modify, and learn!

I have designed/built copper pipe J's for years and have designed a 2m/440 single feed J.  Cheesy
You can search eHAM for my article or go to the files area in:
http://groups.yahoo.com/group/HAM-SATs
http://groups.yahoo.com/group/RDF-USA
« Last Edit: April 15, 2012, 09:55:54 PM by N6JSX » Logged

KF5PGT
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Posts: 38




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« Reply #11 on: April 15, 2012, 09:48:55 AM »

Ok, some experimenting leads me to more questions.

I can recieve very well using the j pole but now I'm questioning my transmitting ability. Here's what I know so far:

-I can recieve full quiet the identification recording on two local repeaters. This is using my 2 meter rig and my HT using the j pole in the attic.

-When I transmit with both radios, I get courtesy tones from the repeaters, except from the further repeater using my HT.

-If I transmit on one radio and monitor with the other, I only get feedback.

-I whipped up a center fed dipole using 12 gauge solid copper wire. I got the same results, but the j pole is clearly a better antenna because I had way more background noise while receiving.

So here are my new, improved questions.

I've always been under the impression that just because I can recieve strong with an antenna doesn't mean that same antenna will transmit strong. Is this basically correct?

If I'm causing a repeater to send a courtesy tone that means I'm reaching the repeater, right?

Once I get it working, I might try drilling a hole and hanging it.
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WB6BYU
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Posts: 12983




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« Reply #12 on: April 15, 2012, 10:10:41 AM »

Quote from: KF5PGT

I've always been under the impression that just because I can recieve strong with an antenna doesn't mean that same antenna will transmit strong. Is this basically correct?



Well, sort of...

Generally you would expect that an antenna that receives strong signals should transmit
strong signals.

You can receive a strong signal but not transmit one under several conditions:  perhaps the
most common is when the other station is running a lot more power, or if the SWR on your
antenna is bad enough to cause the transmitter to shut down.  There are other quirks that
can cause a difference, too.

Oh, and if you don't have the proper access tone set for the repeater, it won't acknowledge
your signal no matter how strong it is.


Quote

If I'm causing a repeater to send a courtesy tone that means I'm reaching the repeater, right?



Yes it does - at least briefly.  If your signal is sporatic, or if your power drops rapidly due to
low battery condition, then you can get a tone but not have a good enough signal to maintain
a conversation.


Quote

-If I transmit on one radio and monitor with the other, I only get feedback.



The radios are too close to each other, and you are working through a repeater.
If you transmit simplex on one radio you should be able to hear the audio on the other receiver.
But when you transmit on a repeater the strong signal you are transmitting on the input
frequency overloads the other receiver and prevents it from hearing the receiver output
frequency.


Quote

-I whipped up a center fed dipole using 12 gauge solid copper wire. I got the same results, but the j pole is clearly a better antenna because I had way more background noise while receiving.



Not necessarily related.

When receiving a signal on FM, the LESS background noise you hear, the STRONGER the signal is.

Note that the J-pole is prone to common mode RF currents on the outside of the coax, making it part
of the antenna.  The dipole can have the same problem, especially if the coax runs parallel to one of
the elements for some distance after it leaves the feedpoint.   The coax acting as an antenna does two
things:  it tends to pick up more interference from electronic devices it passes between the antenna
and the ham shack, and the radiation from the coax may cause additional lobes and nulls in the
radiation pattern.  So, while a J-pole IS simply an end-fed dipole, and should have the same radiation
pattern and gain, installation and implementation differences could cause either to work better in
a specific direction.
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KF5PGT
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Posts: 38




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« Reply #13 on: April 15, 2012, 01:02:55 PM »

A little more experimenting!

I found a repeater on echolink that was kind of far out for my location, but I tried anyway. I dialed up the repeater on my rig and connected via echolink. The repeater is a little over 40 miles away. I was able to hear myself on my rig when I identified via echolink, and I heard myself identify on echolink when I used my rig. When I called via echolink it came in stronger than when I called via my rig. But it came thru. So that tells me two things...

1- my antenna works, but might need a little tweaking.

2- there is not many people monitoring on my nearest repeater, either that or they're jerks.

I appreciate all the help. Experimenting and building stuff is one of the things that I love most about ham radio. It can be frustrating but the more frustrated I get, the more I tend to enjoy the project. Sometimes I make a project more difficult on purpose, just to see if I can do it!

So now I'm gonna tweak my j pole abit to squeeze some more juice out of it. After that, I need to study for my General ticket so I can build a HF antenna!

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VE3SAB
Member

Posts: 7




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« Reply #14 on: June 16, 2012, 08:38:24 AM »

Many years ago I lived in an apartment where the landlady was a real "B" with regards to any outside antenna.  To use my 2m hand held, I simply bought an FM broadcast folded dipole antenna from Radio Shack.  I re-tuned it and used one of those cable tv baluns to match it.  I then taped it to my 2nd floor window.  With 1 watt I could use all the local repeaters and some about 20 miles away.

You could also try that sometime.

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