J Pole in Attic

[stevez97]

I mounted a J Polie in the attic for my Yaesu FTM400 radio. It's being fed with 50' of LMR400. However, I have a 3:1 SWR and having a tough time hitting local repeaters.
As you can see in the pictures, I have a good deal of foil-lined HVAC ductwork in the attic, which I am guessing is causing my problems.

Does anyone have any ideas/suggestions that may improve my situation? This is an HOA community <sigh> thus the reason I'm looking at an attic mount.

My plan B might be mounting the J pole to a remaining satellite dish pole in my back yard, however, I would have to keep the peek height to only 10 or 12 feet in order to remain in "stealth mode." Also finding a way to run the coax from there into the house is something that already gives me a headache. <grin> (The house is on a slab with no basement, so might have to go UP into the attic and then back down.

Thanks for any thoughts, ideas and/or suggestions!

 

[mass-man]

yep that ductwork could be a major problem...move the thing around the attic, and of course as far away from the ducts as you can. You might find a sweet spot where it works pretty good...the Jpole may not be the best for that attic. I simple groundplane might be good to experiment with.

 

[popnokick]

Your j-pole is RF coupled to the metal mast on which you’ve mounted it, causing it to detune. That type of j-pole is prone to doing that. Either get a non-metallic mount or a j-pole type that won’t couple to the mast, e.g. a Dr Fong.

 

[R8000]

Mount it outside like you were thinking. Spray paint the antenna black with the exception of the connector and tell the HOA it's a new style over the air TV antenna and to bugger off. Worst that can happen is that your neighbors will think it's awesome and want one for their TV as well

 

[mass-man]

weave plastic leaves and flowers thru the Jpole outside and the neighbors will compliment you on your green thumb.

 

[Ubbe]

Try and remove that metallic pole and let the j-pole rest on the floor and hang it from a non metallic wire. It will then be positioned below those silver ducts.

J-poles are extremely sensitive to other nearby metallic objects and needs lots of free space. The point where the braid of the coax are connected to the antenna are not "cold" and the coax will be part of the antenna and its routing are crucial to the antennas performance. Sometimes you can see people trying to decouple the coax by having 4-5 turns of coax curled up directly below the antenna. If you can have someone move the coax while you transmit and watch the SWR meter you can check if the coax belongs to the antenna. You adjust SWR by moving both points from the coax up or down the antenna. It's easier if the coax are fastened by hose clamps and when the best SWR are found it can be soldered to the antenna, if needed but not necessary if used indoors.

/Ubbe

 

[littona]

Try and remove that metallic pole and let the j-pole rest on the floor and hang it from a non metallic wire.

What is a non metallic wire? A rope?

 

[vagrant]

If you must use a Jpole in your attic...I recommend 12 or 24 inches of PVC and lower the antenna away from that ducting. Even if the ducting were not there, that slight difference in height is not going to perform a miracle. What would possibly help is the earlier advice about moving it around to different spots and testing. I would still keep it low.

If the SWR improves and you're happy great. I would initially try a Diamond X30 or X50 on a short PVC mast.

 

[MUTNAV]

What is a non metallic wire? A rope?

Not necessarily, a "wire rope" is frequently used with winches and such (and some guys), it's far more flexible than "strand" which is the stuff you usually see guying or stabilizing telephone poles.

Fishing line would be a non-metallic wire, but not a rope (and probably applicable here) or a piece of string (or dental floss?)

 

[stevez97]

Thanks for the multiple replies.

Here's a quick update:
1. Removing the mast from the J-Pole had no effect on SWR or transmission abilities.
2. Hanging the J-Pole (from a rope) and having it at floor level, DID improve the SWR from about 3.5 to 2.5, but the transmission abilities decreased considerably. Previously, I got hit about 8 repeaters (albeit with a fair amount of noise) however moving the J-Pole to floor level now has me only hitting one repeater.
3. I've moved around the location of the antenna, but so far haven't found a "sweet spot."

Also, while up there today, I had an additional "ah-hah" moment (also known as a "DUH" moment,) and noted that the entire roof is lined with the foil-backed foam board, thus further reducing transmission capability.

At this point, I believe Plan B and now become Plan A. I luv the plastic leaves and flowers suggestion! I think that just may work!!!

 

[vagrant]

It is a Diamond VX30 antenna. Using some non metallic paint will disguise it even more. There are other Diamond models as well. The issue with a Jpole is the perpendicular coaxial cable, but maybe you can hide it somehow.

 

[W5lz]

I'll bet you've figured out that mounting any antenna in the attic isn't the best thing for performance. But sometimes that's your only option, you just have to 'live with it'. What's around or over an antenna will affect things more than what's "under" it. That antenna hidden in a plant is a nice idea -IF- the 'stems' aren't wire/metal. Or just solder all those 'stems' together and make that the antenna??

 

[N4GIX]

The vine he is using is plastic, so no problem at all...