Ramsey AM25 Transmitter

Hi Sean,

Welcome aboard. I have one of these units which I retired in favor of something else but I have done quite a bit of analysis and experimenting with it and there are several threads on this board about this transmitter.

Start with this one:

http://www.part15.us/node/1896

You will probably find these helpful as well:

http://part15.us/node/637

http://part15.us/node/938

There are others which you can find by using the SEARCH function. Just enter AM25.

I would be glad to focus on anything in particular you wish to discuss here.

Regarding range, my antenna was a tuned transformer coupled 3 m antenna mounted horizontally on my basement ceiling. Did this since I only wanted to cover my 1/2 acre property so I didn't do ground radials or outside vertical installation. I could hear my signal about .2 miles away on a car radio usable signal and about double that with the signal just above the noise. One of the reasons I retired the unit is that I didn't need this range and was concerned about the above legal input power.

Neil

Thanks Neil

That still pretty good range. I would like to hear more about the tranformer tuned. That may work
way better than what I have going now.

Thanks Sean.

Hi Neil

Thanks for all that information. I have been an engineer for over 22 years. One thing I have notice
about the AM25 transmitter. Even though you reduce the power and modulation get better the effiency
of this design is very low... If you put a 100mw DC input the output is very low. Iam wondering how
effiecent is the SSTRAN 3000 is. I talk to Ramsey Engineers and they meant 100 mw out put power.
Not the input power so the design never did meet FCC Rules. I use 12 volts even,but Ramsey claim to use a 9 volt source and the transmitter will still work,but they do not tell you the Bias if you use a 9 volt source.
I would figure it would be around 4.7 volts that would still put you outside the FCC rules.
If you get this transmitter down to a 100mw DC input the output power is going to be far less than some transmitters. I have an old pair of Radio Shack CB Walkie Talkies they claim to have a 100mw DC input ,and the out put power is 50mw..That mean radio Shack figure out how to get a 50% effiecent Design. Ramsey
Does not even come close. From what I had read on your test. I saw one of your test stating over 400mwDC input but the output did not even reach 100mw...I not putting down Ramsey,but it would be better to find
a 100mw DC input design that could put out at least 50mw to the antenna. I saw you had the Ramsey transmitter down to a 194mw DC input yet it only delivered 32mw output. This is not very good at all.
I think if you reduce Ramsey down to 100mw DC input you may be only getting a 15mw to 20mw output.
Again it may have good audio at that point, but really I thing SSTRAN 3000 may be way more effective in put a signal out. Also it has really good audio. Have you done any test to see what kind of power SSTRAN
put out.

I talk to the engineer at Hamilton that makes the Range Master. He claims it has 100 mw DC input and
put more than 90% of the power to the antenna. He claimed getting 95mw output. with only a 100mwDC input,and if so this is the best design I have ever heard of since I have been messing with electronics since
1975.

Anyways out of all the Kits on the Market is the SSTRAN 3000 the Best???

Thanks Sean

Sean,

My data indicate that the AM25 efficiency is low. A traditional Class C RF amplifier should exceed 70%. As I look at the circuit I wonder if it can well be that it is not operating Class C. A tipoff is that there is no tuned tank in the collector circuit. As you probably know, a Class C circuit operates between saturation and cutoff and switches between these points in the minimum amount of time so the output device is either fully conducting or fully off. I have not scoped the drain voltage or current to determine if this is the case. Nice experiment for someone who is interested.

Regarding the "best" AM part 15 kit, I can only speak for the two I have experience with: the Ramsey AM-25 and the SSTRAN AMT-3000. They each have their strengths and weaknesses. Since I have commented on the AM-25 I will now comment on the AMT-3000.

As a kit, it is beautifully designed and documented and is a great value for the money. The onboard audio processing works great for me. It consistently measures 93 mW final input power every time I have checked and the synthesizer circuit holds the frequency very well.

I have not measured the output power nor the efficiency for lack of a definable suitable load to use for the test. Here's why. This transmitter expects to see around an 800 ohm resistive load at the collector of the output stage. To make this work with an antenna, there are capacitors to ground (one variable) and an inductor in series with the antenna (external loading coil or built in inductors). It is general wisdom that a series loading coil on a short antenna provides the XL needed to cancel the extreme XC from the short antenna. In the SSTRAN, in order to make the Z transformation work, the antenna is tuned slightly inductive (high side of resonance) so the very low resistive component of the antenna and ground system is transformed up to 800 ohms (see the link below for more on this).

This requires that the transmitter is carefully tuned according to the instructions for the particular antenna used. If it is not properly tuned, the output waveform distorts and the audio distorts. If properly tuned, it works like a champ with great audio and a clean looking textbook RF waveform up to 100% modulation.

Here's an extensive discussion on the antenna matching theory for the SSTRAN which includes comments from the SSTRAN designer and manufacturer, PhilB. It is a long read but it is packed with information. In it, I present my analysis of the antenna tuning network using complex algebra. There were some surprises in my derivation and I hope you find it helpful.

http://part15.us/node/883

Neil