Attic antenna: How far from conductive material

[Need2Know]

I've seen commercial antenna writeups that say that the antenna should not be located near "conductive material."

Several questions about this:

What is considered "conductive material?" My attic contains some aluminum AC ductwork, the round accordian type.

How far does "not located near" mean? 1 foot? 10 feet? At what distance does significant degredation begin to occur? And how significant is "significant?"

What is the effect of conductive material at varying distances from the antenna? How does conductive material "nearby" degrade reception or transmission?

I'm preparing to install a J-pole in my attic for 2 meter/70cm.

 

[W5LZ]

N-2-K,
Your question is one that doesn't have an 'exact' answer, or one that is easily measured. It usually means that the reason for the question is one that involves a 'less than optimal' installation of some type, such as in your case. It also means that while there can/will be some affect, and that affect isn't going to be 'good' in some way. But saying exactly what that affect will be and compensating for it, is one'a them "try it and see" thingys (and something all of us go through). Here are some things that you might want to consider.
Antennas would rather be 'above' metal than beside metal, and usually don't like being under metal at all. That "near" is relative. It depends on the frequency of use, and thinking in terms of wave lengths instead of yards, feet or inches. Once you get to a couple of wave lengths the affect of things around an antenna is usually diminished to almost nothing, in general, or to some 'state' where it's not going to make a lot of difference. Basically an "I can live with it" thing. One very rough rule of thumb is a separation distance of the length of the antenna. There's nothing 'exact' about that at all though!
I would think that putting that 'J'-pole above those AC ducts would be 'better' than down 'amongst' them. From there, I think it'd be wise to try it and see if it's behaving half way reasonably, acceptably. Wanna try a different positioning? Why not? Try it and see what happens. Each and every installation is different, so there's no 'sure' way of knowing before hand. If you can accomplish your objective with less than 4 or 5 dozen trips to the attic, I figure you are very lucky.
Good luck.
- 'Doc

 

[Need2Know]

Thanks.

My best analogy to the "dozens of trips to the attic" problem is when I'm listening to a weak commercial FM station in the car and I pull up to a traffic light. As I'm slowing down, the station goes in an out, and I need to creep forward to find a spot where the staion can still be heard. It may be a matter of inches that makes the difference between hearing it and not hearing it. I guess that's what I'm facing in the attic.

I'm wondering also if there are any fluky situations where the proximity of "conductive material" may increase the db of the signal? Conductive material can giveth and taketh away? But unfortunately, I bet the ratio of the giveth to taketh away is probably 1:10,000.

Does anyone have a remote control attic robot I can borrow?

 

[BJ radionut]

N2K: I'll go with Doc...There's as many do's than dont's when it comes to antennas...but with your situation the best thing is to do what is practical and safe.
Then see if it works....it's always easier to make small changes once you have a set-up that is working.
In the case of 2 meters...
I had a old Ringo off the side of one of my towers once, the stand off was only 5 or 6 feet and it "tuned" just fine...was it "omni-directional"...hardly...but it worked...
I have also had 1/4 wave vertical dipoles hung from the rafters before...So put it up and see what happens...
GL
All the Best
BJ

 

[W5LZ]

Can being near metal increase gain? Yes, and no. It can certainly make an antenna directional, so in that respect it increases gain. But there's going to be a lessening of gain in some other direction (*how directional antennas work). How about an increase in omni-directional gain? Not really, unless that 'near' metal is in a very special and specific relation to that antenna. That's just not something I'd ever expect to see.
- 'Doc

 

[Beetle]

Thanks.

My best analogy to the "dozens of trips to the attic" problem is when I'm listening to a weak commercial FM station in the car and I pull up to a traffic light. As I'm slowing down, the station goes in an out, and I need to creep forward to find a spot where the staion can still be heard. It may be a matter of inches that makes the difference between hearing it and not hearing it. I guess that's what I'm facing in the attic.

I'm wondering also if there are any fluky situations where the proximity of "conductive material" may increase the db of the signal? Conductive material can giveth and taketh away? But unfortunately, I bet the ratio of the giveth to taketh away is probably 1:10,000.

Does anyone have a remote control attic robot I can borrow?

What you're experiencing is called "picket fencing" and "multipath". Think of driving down a street slowly, and trying to watch what's going on behind a long picket fence - in some positions you can see things and just a few inches along, you can't see it any more. With broadcast radio, particularly with FM because of the much higher frequency, the signal your radio's antenna is picking up is arriving at different times: directly from the transmitting antenna, reflected off the 18-wheeler 1/4 mile away from you, reflected off that rocky hillside, reflected off that large metal-framed building... (you get the idea). Sometimes two of the signals will arrive very close to 180 degrees out of phase and very nearly the same strength. Thus, they cancel each other out and you hear nothing.

To find out whether the conductive material will help or hinder, experiment. No two antenna installations are quite the same, and the same goes for attics.