What is the impedence of

It is probably designed to drive a 72 or 75 ohm load. This doesn't necessarily mean the output impedance is this value.

Transmitters are designed to drive a specified load but not by setting the output impedance to equal the load impedance. Doing so would mean that half the power is lost in the transmitter. For this reason the output impedance is usually designed to be a very low value in the order of ohms.

The output impedance of the final RF device is primarily determined by the supplied voltage and current. This impedance is then matched to the expected load using an L C impedance transforming filter. In a very low power design power wasted in the circuitry is not a big issue unless the designer is trying to achieve high efficiency such as is done with some AM Part 15 transmitters. For Part 15 FM this is not important since the output power required is very small and the DC input power is not restricted as it is for AM.

This probably over answers your question but perhaps this additional information is of interest.

Neil

I can't seem to find the manual for this transmitter so I am going by memory, if the FM 25 B is tuned correctly it should read 0.5 volts at the TP 1. I was reading that on a digital Vom and wondered if there was something wrong with the tx. I readjusted the tuning coil LC 1 and was able to get a reading of 4.5 volts which led me to believe the rg-6 cable i am using just isn't matching up to the impedence on the output terminal.

Now I am at a loss, could the loading coil be flaking out on me causing a false reading?
I have a spare and could change that out if needed.

The RF output is turned down to yield about 2 mW, the cable is new rg-6 75 ohm 100 ft and the dipole is as simple as it gets, two wires, one on the shiled the other on the hot with a balun in line. Range is just about 800 feet which is fine with me. I just wondered why the voltage would not come up to the suggested 0.5 volts.

P.S. Happy Anniversary to my wonderful wife Stephanie, here's to 8 more wonderful years together.

Barry
Blue Bucket Radio 89.7 FM
The Little Things Radio Show

Happy Anniversary Barry and Stephanie, and I also hope you have eight more.

What do you both plan to do in nine years?

Barry,

I am not familiar with the FM 25B but I do have a FM 25. There is no test point in the output circuit of the 25. There is one in the PLL circuit which is used to check that the PLL is using the proper signal level for the frequency selected.

Do you know what TP1 in the 25B is for? I suspect it is for the PLL and the voltage would not be affected by the antenna loading. I seem to recall that getting the recommended voltage by adjusting the core of the inductor on my 25 was not straight forward and resulted in having the core part way out of the coil.

Neil

Barry, since you own a Ramsey 25B you are entitled to have a manual, so here's what I will do if you answer my survey... I will scan and e-mail you a 25B Manual.

Survey:

YES

NO

But Thanks, Carl or whatever your name is.

Carl,

Yes,
If it isn't too much trouble. I looked on the Ramsey website and seems the .pdf version of that manual is no where to be found. The way my luck goes, after I receive your copy I will probably find the actual manual.

Barry (or at least that's what my mother called me....most of the time.)

OK, Barry. Today I will crank up the scanner and get you a manual.

By the way, the line on the Survey that said: "But Thanks, Carl or whatever your name is", was actually a line on the Survey so that you could thank me by checking the line.

I agree with youyr mother that your name is Barry

I know another Barry. He is our present president.

There are lingering questions whether President Barry is truly our president, in as much as he may never have been born.

Barry, I hope you read this, although in a minute I'm going to see if I have your e-mail address so I can send you Part 1 of the Ramsey Manual.

I am doing the scan in high resolution because the manual is on the tiny side, especially the schematic.

However the shear size of the PDF got so big my computer went on strike, but the solution is to break it into three parts.

Part 1 is ready and I'll send it to you in minutes unless the Lord is not willing.

I have a much better personality and set of values than that other Barry.

As for TP 1 I was thinking it was tied to the LC loading coil, that the FM 25B PLL  would not lock into frequency unless the LC was receiving 5.0 Volts.  Tuning these things is weird because as Neil said, sometimes the slug was sticking out of the top of the coil housing before the transmitter would be happy.

I see a sharp peak that disappears as fast as it shows up when using a volt ohm meter but after more careful tuning (often slow tuning) i would find the sweet spot and the LED would burn steady without modulation and then flicker if audio was at an acceptable level.

Maybe I am wrong about this, I have yet to see that 5.0 volts , it is closer to 4.8 ,4.9.
It could be the LC is worn from changing frequencies so often.

Carl thank you for your help scanning that manual, i looked again today but still could not find it.
[email protected] is a good email address if you haven't already sent the file.
If it was to [email protected] that is fine as well.

The coil adjustment (on the FM25) is used to set the free running frequency of the transmitter signal generation IC so the PLL can pull it on to frequency. It should only need to be changed when making a large change in frequency. The lock light should stay solid on with modulation.

When you receive the manual it will probably explain this.

Neil

Barry, the first 3rd of the manual has been sent to the gmail address.

For some reason the bluebucketradio address rejected the e-mail.

Carl,

I must not have that other email account set up correctly.

Edit: Here is a photo of TP 1 which is very close to the loading coil in fact it is part of that circuit once it gets past all of the resistors. A closer look here