I boughht a 10 watt fm transmitter but antenna

[alcyone]

Jul 28, 2011 6:23:18 GMT -6 RFBurns said:

Odd. Even at 1mW to a ground plane 10 feet into the air should get you a couple of blocks. I know, I have a setup using the el-cheapo 1/4 ground plane on a 8 foot fence pole fed with the 1mW unit and surrounded by mobile homes, the signal reaches 3 blocks on a bad day, up to 5 on a good day after a rain. This is my "fall back" setup for those "don't feel right" days. I suspect you may have a VSWR issue. Even at 1mW a high VSWR will kill that flea signal just as bad as a high VSWR would kill a 1KW signal.

This doesn't get anywhere close to that. I tested it at the full 10 watt power and only got several miles if even that. I downloaded a program to calculate the lenghts of the ground plane and tuned it to my frequencey. Of course, I can't properly test it because i don't have anything to measure the SWR.

rfburns said:

How are you tapping the power after the attenuator, and is the attenuator a "non-inductive" load? The transmitter should operate at normal temperature even running through an attenuator, it is the attenuator that should be getting a bit warm. However with an attenuator inline or not, if there is a high VSWR on the feedline and/or antenna itself, no doubt that will make the TX run hot.

I believe the attenuator is non-inductive. If I run on full power with the attenuator the transmitter doesn't get hot at all, but the attenuator will burn your fingurs if you touch it. Whether it's a dummy load, ground plane or attenuator with ground plane the transmitter doesn't get hot unless the power is scaled back.

If the load, dummy or antenna, is a non-inductive 50 ohm load, or a well tuned antenna, the unit should run fine and sound good. A high VSWR will not only kill the signal coverage, but will also dampen the carrier bandwidth as well as shunt the channel separation by feeding all that back into the transmitter output, then into the driver stages and into the modulator and stereo generation circuits...be it a package IC or separate circuits.

Imagine if we could hear VSWR, much like how we can hear feedback from a microphone too close to a loudspeaker. What happens to sound when feedback is present? It gets "shunted" and overtaken by the feedback. Same happens to RF with a high VSWR and the audio also becomes overtaken by the VSWR, causing it to sound muddy or weak, along with a sweating transmitter getting too hot.

As OER suggested, get a SWR meter and check things. The Workman SWR -3P is a good meter and can be found in abundance on ebay for around 25 mulas.

The finals in those Chinese units have internal thermal and short protection, though the short protection is "short" lived, meaning too long of a short or high VSWR will blow out the final. Too bad they did not incorporate an auto-shut down or crowbar circuit to dampen the power level in the event of a high VSWR. Many operators believe that they can overcome a VSWR issue by simply cranking up the power to compensate, but all that increased RF power is doing nothing but increasing the VSWR ratio, and in turn dumping the increased reflected RF back into the output....POOF! (who let the smoke out...POOF...POOF POOF POOF!)

An improper matched antenna and coax can also cause increased spectrum garbage which will rob your transmitter of power output and scatter that power across the spectrum at the harmonic points.

With a proper match, you can crank up the power and the VSWR ratio should be the same, plus or minus a point or two in proportion to the power output. In any case, when properly tuned, the antenna and coax should present a near perfect load match. Taking the time to obtain that "as close as possible" match is well worth the effort in both signal coverage and sound quality.

Peace!

Thanks for the recommendation on the Workman SWR -3P. I will buy one.

When the ground plane is connected the sound is amazing. Fantastic stereo seperation and quite loud. It rivals that of one of the best sounding radio stations around here. I just had to tweak my audio processing to work with this unit. I tested it on a crappy car radio with crap speakers, so any inperfections stand out like a sore thumb.

[Kage]

Jul 28, 2011 21:22:06 GMT -6 alcyone said:

Jul 27, 2011 21:09:44 GMT -6 Ozone Express Radio said:

Those Chinese units have a thermal protection built into them that actually works (ask me how I know) and I never lost an output transistor to heat. I did zap a couple because of bad mismatches with antennas.

How hot did yours get? I've not pointed my infrared thermometer at the case when it was running, but my 20 Watter is pretty cool to the touch with the fan running while broadcasting, although the heat sink inside is hot enough to burn you and my 1mW unit is cool as a cucumber when running and it isn't fan cooled.

I'd be checking the SWR and see what's going on.

Strangely enough I broadcasted into a wire wound resistor dummy load with an HLLY 5 watt for years (I was told this was like transmitting into open ended coax). The transmitter never burned up.

I did the same thing with a homebrew dummy load until I knew better. Oddly enough with small wire wound resistors it only increases the SWR on the dummy load a slight bit. The bigger the resistor though the more inductive it becomes instead of resistive.
Regardless at 5 watts that's not a lot of hell for a RF output to put up with. Still not healthy for the PA device though.
Start increasing power beyond 5 watts and introduce larger wire wound resistors and tell me you don't pop something into magic smoke

Jul 28, 2011 6:23:18 GMT -6 RFBurns said:

Jul 23, 2011 17:31:04 GMT -6 alcyone said:

The transmitter is power adjustable down to 1 mw. Going straight to the ground plane the signal only went a couple houses down the street.

Odd. Even at 1mW to a ground plane 10 feet into the air should get you a couple of blocks.

Hope I am not quoting out of context but as the old saying goes, all depends on the radio receivers sensitivity.
Some radios can pick up a 1mw signal many blocks away, other radios can't pick up a 10w signal a mile away.