RG58 Antenna Wire Question

**buddhawood** · 10-23-2011, 03:37 PM

This may be a stupid question but my tuned J-pole antenna is about 80ft from where my transmitter is and I have purchased RG58 cables from Monoprice, I was wondering how crossing the some of the powerlines from the controllers would/could affect the signal. Also on another note. I have aluminum siding, I have the antenna 4ft away from the house but can I run the wire along the siding with out any ill effects?

**dirknerkle** · 10-23-2011, 04:41 PM

Wow, an 80-foot run is going to put a big dent in the output of one of these low-power FM transmitters... Even though the antenna is tuned, I think you're going to lose more in the cable than the antenna is going to gain, for a net loss of signal... I doubt the power cables in the yard will have much effect as long as you don't run them parallel with the antenna wire, and try to keep them away from the antenna wire as best you can anyway. The aluminum in the house won't have a huge affect on the antenna wire. I have aluminum siding too, my antenna is inside the garage and I still get a good 1/4 mile distance, sometimes more with a Vastelec FM01.

Any chance you can move the transmitter closer to the antenna and run a longer audio cable to it? I think you'd find that would be a superior solution as there'd be less transmission loss... just an idea...

I'll bet Idunbenhad will chime in here -- a true antenna expert...

**Dennis Cherry** · 10-23-2011, 06:26 PM

Here is some numbers for both RG-58 and RG-8 for 100 feet.

These calculations for Pout (Power out in watts) are for an input of 50 watts.
Cables listed are typical coax cables by Belden.

For 100 ft cables:

RG-8/U, Belden 8214			RG-8X, Belden 9258
MHz	dB/100 ft	Pout	MHz	dB/100 ft	Pout
100	1.7	34	100	3.1	24

RG-8/U, Belden 9913			RG-8/U, Belden 9913F
MHz	dB/100 ft	Pout	MHz	dB/100 ft	Pout
100	1.3	37	100	1.5	35

RG-58/U, Belden 8240			RG-58C/U, Belden 8259
MHz	dB/100 ft	Pout	MHz	dB/100 ft	Pout
100	3.8	21	100	5.4	14

RG-174/U, Belden 8216			RG-59/U, Belden 8241
MHz	dB/100 ft	Pout	MHz	dB/100 ft	Pout
100	8.4	7	100	3.4	23

Note: RG-59 is 75-ohm cable TV coax.

**IdunBenhad** · 10-23-2011, 06:51 PM

Hi:
DING-DONG: DING-DONG

Because the coax is shielded, running it alongside the aluminum siding should have no affect whatsoever. The length of coax is your real worry. As Dirk and others have said, you will be losing at least 1/2 your power and maybe more in the coax. This means you will end up with 15 mw or so at the antenna, but 15 mw can go a long way. My first year transmitter was an Ipod type, like a Belkin, that went for over 1/4 mile with no antenna and just taped to a window. I think it coupled to the aluminum window frame and that served as the antenna.

As Dennis pointed out, if you're using common RG58/U, your loss at 100 mhz will be 3.8 db/100 ft. 3 db loss represents a power reduction of 1/2, so with your 80 feet of coax, you will be dropping your power almost exactly 1/2.

The j-pole being 4 feet away from the wall will probably bother it somewhat. The only way to tell is to measure the SWR (Standing Wave Ratio). Because most DIYCers don't have a SWR meter, the next best thing is just to test the range. If your getting a couple hundred feet each side of your display with a good, noise-free signal, you've got it made.

My bet is you will have plenty of range.

**buddhawood** · 10-23-2011, 11:52 PM

Maybe I'm over thinking this. The reason I am 80ft from the transmitter is because my house sits on the inside of a large curve and I have set up the j-pole at the corner of house so I have a clear path to the whole curve. (the 4ft I have it away from the house out out from the corner) Do you think I would be better off just to set it up in the front close to the transmitter and not worry about the other side? Thanks for the patience in answering my questions.

**dirknerkle** · 10-24-2011, 12:16 AM

Why not fire it up, get in your car and drive around to see what it sounds like? That's the only real way you're going to know...

**buddhawood** · 10-24-2011, 08:08 AM

Originally Posted by dirknerkle

Why not fire it up, get in your car and drive around to see what it sounds like? That's the only real way you're going to know...

Because that just makes too much sence! I'm afraid to say I just didn't think of it. Why look for the simple solution! Thanks! I'll do that tonight.

**Dennis Cherry** · 10-24-2011, 09:08 AM

I do the drive around also as my final check of range and interference.

You can also compare your audio level against a regular FM station, not one of the RAP type stations.

Reducing your audio level, if you are over driving the audio input, will actually get you a slight increase in your range, but stop at the point your audio level is lower than a normal FM station. This will also help in any fuzzy of static on base notes.

As for your coax, watch the cable part numbers, if it does not have the right Cable number and letters (RG-xx l/l) you will not get the performance near what the chart indicates.