Question Concerning The Matchup Of A Ramsey FM Transmitter/Antenna

[Engineer Assistant]

Would the Ramsey 100B FM Transmitter and the Ramsey 5/8 Wave Tunable Antenna operate together efficiently or would their be an impedance mismatch?

[rfry]

The instruction books on the Ramsey site show that the FM100B transmitter is designed for a load impedance of 75 ohms, and that the input impedance of the FMA200 5/8-wave antenna is 50 ohms. So if the two are connected using either 75 or 50 ohm transmission line, then the impedance match and system losses will be somewhat dependent on the length of the transmission line.

This mismatch condition could increase the power dissipation in the output stage of the FM100B -- but maybe not significantly if the FM100B is set for very low output power. Maybe a Ramsey technician could comment on this...?

To avoid that mismatch, a 75 to 50 ohm r-f transformer could be installed at the output connector of the transmitter with 50 ohm transmission line connecting the output of the transformer to the input of the FMA200 antenna.

If compliance with Part 15 is a concern, it should be noted that the TM100B instruction book shows a minimum output power of 5 microwatts (0.000 005 watts) for the unit. The FMA200 antenna book states that the FMA200 has 3.4 dBi gain.

But even if only 1 microwatt of matched power is applied to the FMA200 antenna input connector, the maximum field it will radiate will exceed the FCC limit for unlicensed Part 15 FM, unfortunately. The maximum, matched input power to the FMA200 antenna needed to produce the FCC maximum legal field is only about 8.6 nanowatts (0.000 000 008 600 watts).

The instruction book for the FM100B has some good information about meeting FCC Part 15 FM limits. However an attenuator will be needed in the transmission line to limit the power applied to, and radiated by the FMA200 to about 8.6 nanowatts.

The Ramsey TM100 FM antenna has a 75 ohm input, so using it with 75 ohm transmission line would eliminate the need for the 50-75 ohm r-f transformer. The TM100 book doesn't specify its peak gain, however by calculations it should be about 1.25 dB less than the FMA200.

A 1.25 dB gain reduction in the antenna would mean that the input power limit it needs to just meet Part 15 FM would be about 33% greater than for the FMA200 -- or about 11.4 nanowatts (0.000 000 011 400 watts), which is a lot less than the FM100B can be adjusted to produce. So an external attenuator would still be needed.
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[XRQKFM]

As long as you tune the Ramsey 5/8 Wave Tunable Antenna for minimum SWR; then you won't have a problem connecting it to the FM100b. In a pinch: I used a 75 ohm antenna & cable system; the SWR was still well within reason.

Steve
www.radiobrandy.net

[rfry]

Originally Posted by XRQKFM

I used a 75 ohm antenna & cable system; the SWR was still well within reason.

But that would only be as expected, because the FM100B is designed for a 75 ohm load.
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[RDEngineer]

I have no idea where the notion that the FM100B output impedance is 75 ohms came from. I cannot find any reference to 75 ohm load as either a specification or miss print in any of our manuals, either on-line or in the printed manuals which is where the on-line manuals are directly derived. It is designed for 50 ohms. Both the TM100 and FMA200 are 50 ohms and both will work with the FM100B, or any of our other hobby transmitters which are all designed for 50 ohm output impedance.

[rfry]

Originally Posted by RDEngineer

I have no idea where the notion that the FM100B output impedance is 75 ohms came from. It is designed for 50 ohms.

According to the online User Manual for the FM100B, an "F" connector is used for its r-f output port.

F series connectors are designed for a 75 ohm coaxial environment (see http://www.amphenolrf.com/products/t...75C006375E17F& ). That is the reason why it would be logical to readers of that manual that the FM100B was designed for 75 ohm loads.
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[XRQKFM]

If you look at the FM100b pictured in Ramsey's AM -FM Hobby section; it is clear that a BNC connector is used. Ramsey has used an F connector on the FM25b, so those who don't have access to 50ohm cable; could use the more commonly available 75ohm without any harmful side affects.

I have had other transmitters that used F connectors, like the old Panaxis; even though the transmitter was 50 ohms.


Steve
www.outlawradio.us

[RDEngineer]

The FM100B has a BNC connector. The FM25B does use an F connector but the impedance is 50 ohms. Yes that is a 75 ohm connector but the mismatch created is not significant for the application. Once again all our transmitters are 50 ohm output impedance.

The original question was "Would the Ramsey 100B FM Transmitter and the Ramsey 5/8 Wave Tunable Antenna operate together efficiently or would their be an impedance mismatch?"

The answer is yes. Note that you must remove the whip antenna when using the external antenna.