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gareth
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| Posted: 16 June 2010 at 5:42am | IP Logged
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I have some rg6 coax cable, similar to what sky use with a thick core, its certainly a lot thicker than what I currently have on my fm transmitter. If I swap to this rg6 will it improve range due to less power loss in the cable? Also using a dipole designed for use on 88mhz and mouted verticle, is verticle ok, or should I be using horizontal? I am also getting the best range where I point the antenna, how to get it transmitting in all directions? Different antenna design?
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RadioTech
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| Posted: 16 June 2010 at 6:25am | IP Logged
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RG-6 is no good for FM broadcast. It's 75 ohm cable, for television reception!
If you want good quality coax for FM, you should be looking at LMR-400. It has low loss characteristics and is the right impedance for the job.
If you use RG-6, not only will you lose a lot of power, you will overheat the final in the transmitter, and it could eventually fail.
A dipole antenna needs to be mounted vertically for best reception by listeners.
Yes, dipoles have a radiation pattern which casts more energy in the forward direction than behind the poles.
If you want equal energy radiated in all directions you need to change to something like a 5/8 whip antenna, tuned for 88 MHz or whatever your TX frequency is.
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gareth
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| Posted: 16 June 2010 at 7:17pm | IP Logged
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I am currently using RG58, will there be any difference using LMR-400 ?
Thanks
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RadioTech
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| Posted: 19 July 2010 at 4:09am | IP Logged
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The short answer is YES - there will be a difference.
You won't really hear it on the air, but your transmitter will thank you for it and your signal will undoubtedly reach more people.
Depending on what kind of antenna you have, you may be radiating far less power than you are allowed.
RG-58 is quite lossy at 100 MHz - 0.165 dBm per metre, double the loss of even RG-213 which is not the world's greatest cable.
LMR-400 on the other hand has a loss figure of 0.036dB per metre. It's not the cheapest stuff, nor the easiest to work with, but it is the best cable for this application.
Edited by RadioTech on 19 July 2010 at 4:10am
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gareth
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| Posted: 25 July 2010 at 2:03am | IP Logged
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Thanks, I have already connected up 10m of LRM-400, not really noticed any difference. I think I need to get my antenna up higher as we are in a slight dip, but for that im going to need a much longer cable.
If I go back to using RG58 (being easy to work with) and add 50m to get antenna up high, how much power loss would occur? Would it be possible to use a RF amp from aareff.com set at very low power to counteract the coax loss? So end result is still at the legal 1w
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RadioTech
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| Posted: 26 July 2010 at 8:22am | IP Logged
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As I said, you probably won't notice much difference locally in terms of your signal when using LMR400, but there are noticeable technical effects which are going on.
As for height, if your antenna is in a dip, you're basically wasting your time. Height is might when it comes to FM. You need it as high as you can get it, to get good coverage.
This is why commercial stations put their transmitters on the tops of
mountains, rather than on the top of their building in the middle of town.
50m of RG-58 will lose around 8dB of power - or around 15% of what you started with.
In this case if you feed 1 watt in, you'll get maybe 150mW at the other end.
It's pretty lossy stuff - hence why I suggested using LMR400 and the golden rule of keeping your coax as short as possible, no matter what cable you use.
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gareth
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| Posted: 31 July 2010 at 4:57am | IP Logged
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Thanks, I see im just wasting my time trying to run a transmitter from where I am, will have to find a location at a high point, if anyone in West Auckland is willing to host my transmitter, pref Waiatarua, please pm me, will pay for power costs etc...
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New Zealand Low Power FM Radio Broadcasting Forums : General LPFM Discussion
Topic: power loss in coax
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