AM25 transmitter

[Mark]

Have been considering this one with the one watt option(jumper) since the AM band has so much more empty space but unlike an fm transmitter where you just change frequency to where you want to go, here everytime you change frequency you have to rebuild the LPF and realign the transmitter!!. This seems impractical and is stumbling block to getting this transmitter. Can anyone at ramsey or any one else advise me why it's like this and what do you do if you want to take transmitter to other location where frequencies may have to be changed?

[XRQKFM]

Mark, even the big boys transmitters have to be re-tuned when the frequency is changed or a change in the antenna is made. Even state of the art AM transmitters need to be re-tuned the old fashioned way; regardless if they have PLL tuning. Same goes for the Procaster and Rangemasters certified transmitters; both need retuning when ever a change is made in the antenna or the grounding.

Nature of AM broadcasting.

Steve
www.outlawradio.us

[Mark]

Thanks, It's not the aligning but the rebuilding of the LPF each freq. change that is stumbling block.....seems like I'll stick to FM where it's simply the push of a button and a little change in the antenna length..nothing more.

[radio8z]

The need to retune has to do with the percent change of frequency on AM as compared to FM. For example, on FM the frequency difference between the top of the band to the bottom of the band is (108-88)/100 = 20%. On AM the percent change is (1700-540/1000 = 116%. These figures are not exact but they illustrate that percentage wise a change in frequency in the AM band is a much larger percentage than a change in the FM band.

Another way to state this is that if the transmitter is operating at 1600 kHz the output filter needs to be set to pass this frequency. If the frequency is changed to 800 kHz without changing the filter then the second harmonic (1600 kHz) will also pass which defeats the purpose of the filter.

Small changes in frequency won't require changing the filter so this transmitter has the ability to operate over a range of frequencies with a particular output filter as shown in the manual. Certainly, one could find an unused frequency in each of the frequency ranges so if the transmitter is built for one of the ranges shown in the manual the frequency can be changed within this range without changing the filter or retuning.

The AM-25 I built was built and tuned for the high frequency range (1350 to 1710 hHz) which is adequate range to find a quiet frequency. At my location there are many more open AM frequencies in this range than there are open FM frequencies.

Neil

[Mark]

Thanks for info.
Very informative, When reading manual on ramsey's web site it seems that any change in frequency also needed different components in LPF.....If I have a set frequency here at home, then go to cottage on weekend, have to change frequency there and then come back Sunday night and change back as I understand I have to be rebuilding the LPF constantly!! and have a supply of components on hand...not to practical compared to FM where you just change frequency, nothing more except a little change in antenna length.

[radio8z]

Mark,

I am not sure if you followed what I posted or maybe you are saying what you previously thought. My point was that there are frequency bands of operation for the AM-25 and frequency changes within each band require no changes in the LPF. Using the example I posted, I can operate anywhere within the frequency range of 1350 to 1710 kHz without doing anything except changing the frequency setting switches.

The only time you would need to rewire anything is when going from one frequency range to another such as from 1550 kHz to 1200 kHz. Nothing need be changed if going from 1550 kHz to 1440 kHz since both of these are in the same range for the high LPF.

While not trying to talk you into or out of anything I want you to know your options.

Neil

[kc8gpd]

neil,

what was all the issues with the am25? i can't remember? was thiking of getting my hands on one and jumpering it for 1 watt to do carrier current.

iirc there was also a mod to boost it to 5 watts. any idea's?

at 99.00 i may have a better chance of getting one than any other tx. and if i can use my scrap parts here to get 5 watts out of it that will be plenty to do carrier current here.

in other words i would use it as a building block for a home brew c-c tx.

[radio8z]

Robert,

The AM-25 is a good transmitter and is not as "picky" about the antenna load as some I have seen. There is no need for retuning when the antenna is changed. It is simple to set up and operate.

These may or may not be "issues" depending upon one's point of view but here is what I know, in summary, about the AM-25:

The unit will work with a wide range of antenna resistance. I tested it with a resistive dummy load from about 20 to 100 ohms (or thereabouts...going by memory) and found maximum efficiency is about 30% at 23 ohms. It will work well with a resonant base coil loaded antenna system.

If adjusted according to the manual the unit has an input power greater than the 100 mW used under part 15.219. Perhaps it can operate under part 15.209 but I have no way to affirm this. It can be throttled back to 100 mW final input by changing some resistors if one wants to do so.

When adjusted and operating according to the manual I have observed that Q9 runs hot enough to discolor the PC board. I have also observed discoloration of one of the output network toroid cores.

Previously, I reported that the modulated RF waveform was distorted but this was traced to a defective RF coupling capacitor in my unit. Replacing this cap. fixed the problem and the waveform appears as it should on a scope.

The maximum modulation I measured was 90% with the waveform soft limited on the negative audio excursions. With the audio properly adjusted, this is not a problem.

Someone posted a link for modifications for high power operation and I have lost the link but a net search should enable you to find it. The article also mentioned using a diode in the modulation circuit which may allow for 100% modulation but I haven't tried this. I have no experience with the high power operation so I can't comment on this.

The AM-25, as well as being a good transmitter when used as intended, is a good experimenter's transmitter since it is easily modified. The big issue for high power operation (over 300 mW input power) is the heating of Q9 but it can be off board mounted on a big heatsink. I put mine in a metal box (same as the one I once sent you) and insulated and heatsinked Q9 to the box. It ran just fine this way, just don't make the leads too long.

If it is operated in a room temperature environment the frequency is stable with little drift over time. I have no data on frequency drift in other environments.

The output filter network should attenuate harmonics well but I have not confirmed this beyond observing the RF waveform on a scope and it appeared to be a good sine wave.

I don't know how it would perform in a CC application but it would be worth experimenting with to find out. The usual caution about line voltage hazards applies and I would start with "neutral injection" to avoid exposing the unit and yourself to 120 volts. Be very careful if you try line injection since the capacitors in the output network will not survive line voltage. Personally, I wouldn't trust anything other than a properly designed and fused RF transformer circuit for this.

The AM-25 design is robust in terms of operating with various loads and it should be a good transmitter to use as a start for what you mentioned but watch for component heating at high power. It is also a good transmitter to use for hobby broadcasting with just a simple wire antenna.

Neil

[kc8gpd]

i have a lpb tcu-30

[kc8gpd]

ramsey should consider a 5 watt AM25 maybe with CQuAM and a matching carrier current coupler kit.