View Full Version : 75 Ohm Vs 50 Ohm issues
10-08-2002, 11:23 AM
I was given the FM100B assembled with the export configuration for 1 watt installed, alongside a TM100 Antenna, as a birthday gift. It uses 75 ohm cable, but I didn't know that at the time I purchased 100 feet of RG8U-Foam cable (which is 50 ohms). I understand 50 ohms is the standard for RF. Therefore I have 4 questions:
1) Will using the 50 ohm RG8/U cause damage to the unit?
2) If no damage is likely, how much signal loss is expected?
3) Will using a 50 ohm amplifier cause additional technical problems?
4) Would putting say a BNC or PLL connector on the end of the TM100 Antenna instead of the F connector would help or make things worse?
Any help would be greatly appreciated.
NOTE: I don't live in the USA ;)
As I understand it, the Ramsey transmitters intended for microbroadcasting tend to work fine at either impedance. I believe that 50 ohms is actually the correct impedance - but the power is so low that 75 ohms works fine too.
As a point, consider the two popular antennas Ramsey carries - the 75 ohm TM100 and the 50 ohm FMA200 [I think I remembered that model designation correctly] Either will work fine with your transmitter.
So, 50 ohms is the best match for your transmitter, and 75 ohms is the best match for your antenna, given that you have the TM100. I suspect that if this was at all critical to the reliability of your transmitter, Ramsey would have set a standard by now. The worst that is possible may happen is that you may waste some of your watt. Which is probably just as well, since you are likely to exceed the law with this combination. Be very careful! You might want to consider removing the 1 watt modual and running at 25 mw to avoid a hefty fine, confiscation of all your microbroadcasting equipment etc.
Just saw that you don't live in the USA, as such you may not need to worry about the watt. In very rare exceptions its OK in Canada, but it is rare. Other countries may have exceptions that you may fall into too.
10-11-2002, 05:33 PM
I use RG-8/u, I run about 50 feet of the stuff to a homebrew 5/8 wave antenna on my roof. The stuff is lossy but 75 ohm cable TV coax is much worse. My FM-10 is in metal box, and the antenna output is a panel-mount SO-239 connector, as is the antenna input.
With the SO-239 (Female) the cable ends use a PL-259 (male).
These connectors maintain the 50 ohm impedance fairly well at low power levels, say up to 100 watts.
My suggestion is to replace the "F" connector and use SO-239/PL-259 combinations throughout your RF connections. Don't use the "crimp-on" types available at radio shack. They are very cheap and are succeptible to corrosion/water infiltration.
Also available are Amphenol style ones which are ok although the real Amphenol brand ones are much higher quality.
Stick with the RG-8... it's much less lossy.
As I recall (too lazy to redo the velocity factor calculations) the loss at 1 watt at my frequency (106.5) is around 1.7dB with RG-8/u (the mini style, 1/4 inch diameter).
10-11-2002, 08:56 PM
Thanks for the help Fletch.
10-15-2002, 10:49 PM
We have the same problem with a FM100 transmitter.2 blown finals in less than a year.Ramsey said to use 75 ohm coax with the
tm100 at first and now is saying 50 ohm coax must be used.If the transmitter wants a 50 ohm load Why even use all the 75 ohm connectors??
11-25-2002, 09:54 AM
You'll NEVER know the difference between 75 and 50 ohm cable (and NEITHER will your transmitter!!) The SMALL mismatch is not noticable in your signal and would NEVER be the cause of blown finals!! And the difference in your signal would be less than 1/2 of an "S" unit!!
So use whichever you have or can get reasonably.
vBulletin® v3.8.5, Copyright ©2000-2013, Jelsoft Enterprises Ltd.