Line Loss

[KI4HWJ]

I am just about to install a Diamond X-50A at my station. It will be mounted about 30 above ground on a wood telephone pole for arena lights. Unfortunately, the pole is about 110 feet away from my shack. So I would need about 145 feet of line in total. My question is will I have significant loss of preformance on the 2 meter band versus the line being about 50 feet, but having the antenna being mounted near a metal roof, closer to the ground?
Thanks
73's

[N4LI]

Well, that's sort-of a loaded question...

What type of feedline are you planning to use?  If you're planning to use 7/8 hardline, you might get a different answer than RG8.

What do you mean by "near" to a  metal roof?  Will the antenna be blocked by the roof?  Will it be in a position that the metal might de-tune the antenna?

The X50a only has about 4.5 db gain (the Diamond site didn't say if that's dbd or dbi; but since my V2000 lists its 6m gain as 2.15 db, I am betting we're talkin' dbi).  But, you may find all that gain erased by your feedline.  Ouch.

How much closer to the ground?

It sounds as if no matter what you decide, it will be a compromise, so maximize what you can.  I have a Diamond V2000 in my third-floor attic on a 50' run of 213 for routine repeater use, etc., and it seems to work fine.

My gut tells me to go shorter, but with limited information, I might be wrong.

Peter, N4LI

[WB2WIK]

30' above ground isn't very high.

How high would it be if it were closer?

145' of coax at 144 MHz is not a big deal; every transmission line I have feeding my antennas at home is in the 175' to 225' long range -- a lot of them.

I use low-loss coax at 2 meters (LMR400 or equivalent) and the height advantage vs. line loss in my case very easily tips the scales towards, "higher up, but farther away," so I can put antennas on my tower.  They work way better up there than closer to the shack but on my roof.

But again, 30' isn't very high.  If you can be nearly that high by placing the antenna a great deal closer, that might be worth it.

WB2WIK/6

[KI4HWJ]

Sorry I wasn't specific enough. If I can afford it I would like to purchase as low loss line as I can afford (RG-8 LMR400?). If I don't mount the antenna on the telephone pole, and mount it near the metal roof, it would probably be about 15' off the ground. It would be above the roof, but right next to it. The roof would be sloped towards the antenna, and I worry this may detune the antenna, but I'm not certain. The program for calculating line loss, TLW, tells me I will have 2.099 dB of loss, and with the antenna having 4.5dB of gain on the 2 meter band, this is pretty significant.
If I mount the antenna on the telephone pole, the coax will be buried in a trench, run through ABS conduit.
I now hope to mount the antenna near the roof and being only 15' lower, and having not nearly as much loss, I think mounting the antenna lower, but closer to the radio and having shorter line would be my best bet.
Thanks
73's

[WB2WIK]

I don't understand why the roof position cannot be increased in height.  That's what masts are for.

If you have a place you can mount the antenna that's 15' above ground, can't that mounting position be made higher by simply installing an appropriate, longer mast?

What about the "peak" of that sloping roof?  Can you install a tripod there and get some extra height?

Even with the 30' pole, my first thought as a ham would be "30 feet is okay, but 50' is better," and I'd be looking at ways to add 20' to the 30' pole.

Not rocket science, and extra height makes more difference in VHF performance than any other factor.

WB2WIK/6

[KI4HWJ]

I have decided to just use a tripod on the peak of my roof, since this seems to be the most cost effective method and there would be very little loss in the line. I still live with my parents as I'm only 15 and do not have the support or permission from my parents to put up a 50' mast as I would like to do.
Having said that, would I have to worry about the metal roof detuning or having adverse affects on the antenna?
Thanks
73's

[N9DG]

That sounds like a winner of plan to me. The feedline will be quite short and the antenna will presumably be above most nearby objects. And since the antenna will be at the peak of the roof the fact that the roof is metal won't hurt a thing. In fact it might even help its performance since the metal roof will look like a very large ground plane.

And as a matter of safety be sure that the antenna is well away from any overhead electric power lines. And install it in such a way that it cannot ever come in contact with power lines either while in use or during the process of installing it.

[WB2WIK]

>RE: Line Loss  Reply  
by KI4HWJ on January 17, 2006  Mail this to a friend!  
I have decided to just use a tripod on the peak of my roof, since this seems to be the most cost effective method and there would be very little loss in the line. I still live with my parents as I'm only 15 and do not have the support or permission from my parents to put up a 50' mast as I would like to do.
Having said that, would I have to worry about the metal roof detuning or having adverse affects on the antenna?<

::Absolutely, positively NOT.  The roof could be solid metal and directly beneath the antenna and it will have no effect at all on this particular antenna.  On a horizontal antenna, that would be a different story.

-WB2WIK/6

[KI4HWJ]

Thanks very much for the input and advice, it is much appreciated.