HFU HF Underground
Technical Topics => Part 15 AM and FM Station Operation => Topic started by: dss56 on August 03, 2022, 0138 UTC
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Hi all hope this is the correct place to ask this question.
Getting ready to start an FM 88.5 radio station.
I built an adjustable dipole that will do 108-87mhz with easy tuning.
My question is what coax should i use. I knowrg213 is good but expensive 100ft it like $200+.
Can I use rg6 or some good small coax?
Transmitter is 25 watts adjustable and hope to use only 15 watts or less to cover a 2 mile radius.
Any suggestions on some small good coax/
thanks
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With the output power you are running, your determinating factor will be more based along the length of coax between the transmitter and the antenna. Coax has a loss factor which is measured in dB per 100 feet at various standard frequencies. Although RG6 may be tempting, it is 75 Ohm coax vice 50 Ohm coax. Your transmitter and antenna will have a 50 or 52 Ohm impedance therefor you will need to match the transmission line to suite the equipment.
Some choices for common coax (with rounded figures):
RG-58 - 5dB of loss at 100 MHz;
RG-8X - 3.4dB of loss at 150 MHz; and
RG-213 - 2.1dB of loss at 100 MHz.
If you can pick-up some used LMR cable, that would be the best choice.
LMR-240 - 3.0dB of loss at 150 MHz (this is the size of RG-58); and
LMR-400 - 1.5dB of loss at 150 MHz (this is the size of RG-213/RG-214).
dB Conversions:
1dB loss means about 21% of the signal is lost;
2dB loss means about 37.5% of the signal is lost;
3dB loss means 50% (half) the signal is lost;
6dB loss means 75% of the signal is lost; and
10dB loss means 90% of the signal is lost.
Hope this helps somewhat in making the right choice.
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Agreed. For runs under 50 feet, lmr240 is fine. I use it for short cellular jumpers without issue. Anything longer though, I'd use lmr400.
+-RH
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With the output power you are running, your determinating factor will be more based along the length of coax between the transmitter and the antenna. Coax has a loss factor which is measured in dB per 100 feet at various standard frequencies. Although RG6 may be tempting, it is 75 Ohm coax vice 50 Ohm coax. Your transmitter and antenna will have a 50 or 52 Ohm impedance therefor you will need to match the transmission line to suite the equipment.
Some choices for common coax (with rounded figures):
RG-58 - 5dB of loss at 100 MHz;
RG-8X - 3.4dB of loss at 150 MHz; and
RG-213 - 2.1dB of loss at 100 MHz.
If you can pick-up some used LMR cable, that would be the best choice.
LMR-240 - 3.0dB of loss at 150 MHz (this is the size of RG-58); and
LMR-400 - 1.5dB of loss at 150 MHz (this is the size of RG-213/RG-214).
dB Conversions:
1dB loss means about 21% of the signal is lost;
2dB loss means about 37.5% of the signal is lost;
3dB loss means 50% (half) the signal is lost;
6dB loss means 75% of the signal is lost; and
10dB loss means 90% of the signal is lost.
Hope this helps somewhat in making the right choice.
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Looks like my best choice will be the rg213
thanks for all the helpful info on the coax.
thanks to all who replied.
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To be honest, if you look around eBay you can find some new, pre-terminated (with connectors) LMR-400 at pretty much the same price as RG-213. Keep in mind that true LMR is manufactured by TIMES MICROWAVE. Anything else is a knock-off.
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I've used the off brand lmr cable with good results. I've been using off brand half inch super flex on projects lately also without incident.
+-RH
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I have used LMR cable in the past. The newer cheaper brand cables are working just as good.