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Author Topic: Two Antenna questions - distance from roof and height vs lingth of coax  (Read 1613 times)
K8ASY
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Posts: 4




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« on: December 06, 2012, 04:13:42 PM »

Hi,

Newbie here with a couple Q's.  I've got a 2m/70cm HT and want to get a better antenna for base station use.  It seems a dual-band J-pole would be just the ticket.  My questions:

1) My house has a metal roof.  What distance from the roof should I mount it vertically and/or laterally?  It's unlikely I'll be able to get much higher than the roof; what about mounting on a wall, some distance below the roof?  Or would I get better results from a design that uses a ground plane and mount it on, or close to, the roof?  The house is two stories above ground on one side, three on the other.

2) Somewhat related:  I know the higher I mount it the better (height above average terrain).  However, the higher I mount it, the higher the coax losses.  I can calculate transmission line loss, but how do I weigh in the effect of height?  At what height are your losses from the line going to be higher than your benefit from getting the antenna higher?

Thanks,

Kelly
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WB6BYU
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Posts: 13026




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« Reply #1 on: December 06, 2012, 04:38:02 PM »

Quote from: K8ASY

1) My house has a metal roof.  What distance from the roof should I mount it vertically and/or laterally?



I'd certainly try to get it above the top edge of the roof, otherwise the roof will block it
in some directions.  A mast secured to the wall that gets it just up over the edge will
be adequate.



Quote

2) Somewhat related:  I know the higher I mount it the better (height above average terrain).  However, the higher I mount it, the higher the coax losses.  I can calculate transmission line loss, but how do I weigh in the effect of height?  At what height are your losses from the line going to be higher than your benefit from getting the antenna higher?



Height trumps coax loss in almost all situations.

For example, going from 20' to 50' increases the 2m signal strength about 8dB (at least
for a particular set of conditions that I modeled.)  Even using RG-174 coax (which is
about the lossiest type in common use) the coax loss in 30' of coax would be about
3dB on 2m, leaving a 5dB improvement.  RG-58 would have less than 2dB loss, for a
6dB gain.

That's not to say that you shouldn't choose a better feedline when it is available, but
that it would require a VERY lossy feedline to overcome the advantage of the added
height. 
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N4CR
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Posts: 1652




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« Reply #2 on: December 06, 2012, 04:44:58 PM »

Regardless of the antenna or it's gain, height is EVERYTHING for those two bands. Period. No exceptions given any reasonable antenna.

Antenna gain has very little to do with how well your VHF and UHF antennas will work on your house. Ground plane antennas are easy to build and don't have any of the bad habits associated with J-Pole antennas.

A J-Pole is an end fed zep antenna. Like any end fed zep, it will try to couple into everything around it rather than radiating a nice clean pattern. This means that not only will your pattern likely be not omnidirectional, but some of it won't be vertical, and all repeaters are vertically oriented. Also, everything it couples into will likely be lower in height and will radiate power closer to the ground. J-Poles are expensive to build if you use copper pipe and they come with a lot of self inflicted issues. Best to avoid them altogether.

Concentrate on getting a compact 1/4 wave vertical up as high as you can get it. Spend any money you have to spend on getting it higher. That's what actually makes a difference.

I use a 1/4 wave antenna I built on an SO-239 connector. (actually I use N connectors) Costs almost nothing and can be made practically invisible. You can get 3' brass 1/16 rods at Ace Hardware 2 for $2.50. A few of those and some solder and you're off to the races.

A 1/4 wave 2 meter ground plane is a 3/4 wave 70cm ground plane with a slightly higher take off angle. Remember when I said gain isn't as important as height? You can build a 2 meter nearly invisible antenna for under $10 that will work both bands just fine.

If you're into buying and not building, a Diamond X-50a is an excellent performer. the Comet GP-3 is another excellent over the counter antenna. The Workman UVS-200 is the low cost contender that is essentially the same antenna but about half the price.

If you see a UHF VHF dual band antenna with no radials, it's likely a J-Pole even if it doesn't say so.
« Last Edit: December 06, 2012, 04:52:32 PM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N8CMQ
Member

Posts: 353




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« Reply #3 on: December 09, 2012, 02:33:06 PM »

I have a quarter wave 2 meter antenna with four radials drooped at 45 degrees
5 feet over a metal roof on the second story of my house.
I am feeding it with RG8X and the signal in and out is decent for 148 MHz.
Unfortunately, I lost my notes from when I modified and installed the antenna.
The antenna was a commercial aircraft base antenna (118-136 MHz) and was too long
for ham use, so I cut the radials and main element for 2 meters.
The coax was surplus I had stored away, and I measured the power into the
coax and into the 50 ohm load, but it had to be good, otherwise I would have used new.
It also works as a receive antenna for the Public Service band up to 170 MHz.
It is mounted on a mast held by a tripod, and it has been up for a number of years now,
and the SWR has not changed since I put it up.
 
 While you may use a J-pole, I would suggest a quarter wave, and if you need dual band
operation, there are a few designs for such a vertical out there to try.
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KQ6Q
Member

Posts: 965




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« Reply #4 on: December 10, 2012, 07:45:47 PM »

If you have a metal roof, and it's steel, just get a good dual-band mag-mount antenna and use that,. If the roof is steeply angled, put a bit of a bend part closest to the base to make it more vertical, and put the mag mount as close to the peak as possible.
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