I am trying to ID this fm transmitter i just ebayed. I have had a ramsey fm100 conncted to a Comet on my roof- this never gave me more than a few blocks of signal. This is way more powerful, but i am concerned that it may interfer with neighbors, my home wifi network will not work when it powers up - and it sends a signal for about 2 miles - Can anyone tell me anything about it? I have an swr meter and absolutely no idea how to use it. Are there anyonline tutorials that might show me? Sorry for all the noob questions, but i am what i am. - dog
Looks like it could be a homebuilt job. The 2SC1970 output transistor is quite commonly used in homemade transmitters and gives around 1 watt of output power which with the right antenna could definitely get you out a few miles.
SWR meters are pretty easy to use. You simply connect it between your transmitter and antenna which is almost always labeled on the back of the meter. Normally there is a switch with the labels REF and FWD. You flip it to REFerence and turn its nob until the meter reads maximum, then you flip it to FWD and it will then tell you your SWR. You want as low of an SWR as you can possibly get. Anything over 3 is bad news.
Make sure it's a meter designed for VHF usage. A CB radio type meter will not read correctly for the high frequencies FM broadcasting uses.
Looks to me like a modified something over to the FM band. What is the lower portion that is barely visible in the above photo...the LPF?
As to the jamming effect of the wi-fi, this is a sure sign of major harmonics spitting out of that thing. A properly built and tuned FM transmitter should not knock out your wi-fi system.
I would keep that thing off the air until you can get it tuned up properly and checked for spurious emissions, correct as needed. Heck re-dressing of the wiring and layout of those modules would probably fix 99 percent of the issues.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
Ah yes that must be the power amplifier stage. Definitely running more than a few watts by looking at that, and if I am correct then yes you definitely need to be filtering that output with a LPF (low pass filter) or possibly even a band pass.
If it's knocking out your WIFI then it's definitely spitting out high frequency interference. It could be possible though that there is just so much RF around your room that it is effecting near by electronics like your WIFI. I have seen electronics fail near by powerful transmitters even when the transmitter is tuned and working perfectly. Either way you do need to filter your signal to make sure without a doubt that it is running clean. Running a dirty signal is the easiest way to get noticed in a bad way causing a visit from the beloved FCC
Also you need to make sure your antenna is running with a low SWR itself since a high SWR can cause the transmitter to run hot and possibly generate more interference since the transmitter is not "seeing" a proper load.
If you're confused over these basic things then take a step back and read lots of learning material on basic VHF transmitter operation. Older ARRL handbooks can teach you a lot, even though they are wrote for ham radio enthusiast the technical chapters on VHF/UHF transmitting is the same.
Feel free to ask any questions here and read up on other threads on this forum.
One thing is for sure, that amplifier deck has a lot of sections to tune up. The real problem I see is the lack of loaded coupling between the modules.
Loaded coupling each module's input and output makes it 100 times more easier to tune up each module to the next, while minimizing module interaction during the tuning process.
The last picture shows the final module, and as can be seen, it is equipped with a LPF (Low Pass Filter). However the filter is only a 3 pole, meaning it is probably only good out to the 3rd harmonic and passing everything else above that. To be effective for this amplifier module, it should have been designed with a minimal 7 pole filter, 12 is optimum.
High power requires more pole filtering in the output before anything hits the antenna. I usually recommend at least a 7 pole filter on systems capable of up to 250 watts and 12 pole filters on anything higher than 250. As more power is pushed from the amp, the larger in amplitude are the harmonics as well as the main carrier. The filtering must be able to attenuate those high powered harmonics especially beyond the 3rd, where harmonics will bring more attention than you would care for.
Just add an external LPF to the output, and modify the inputs and outputs of the modules, with the exception of the final module's output, with loaded coupling so as to make tuning those stages easier and make the entire amplifier more stable and less prone to stray inductance reactions from temperatures and environment effects upon the antenna.
I also recommend replacing that Comet with a J-Pole or two stacked folded dipole bays. If that is the amplifier deck I think it is...it is capable of throwing your signal from a J-Pole about 12 to 15 miles at 50 feet up.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
My thanx for the complete and detailed replies- I've been using projects like this as therapy after a stroke 4 years ago - I've been trying to do more demanding tasks, including some circuit board soldering - It looks to me that i can learn a lot just cleaniing up the various components of which, this is the last - i have no idea how many volts it is putting out. - again thanx for all the homework - i'l check in as i go - dog
The modules themselves look great. Some of the excess solder on the in's and out's could be removed, and smaller interlinking coaxes with enough length to dress those and power wiring so they are secured away from vital areas of each module. This will also help keep them from causing stray inductance effects around those tuned circuits.
With a little clean up and reorganizing of things that looks to be a fine rig!
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
Well, as expected, the SWR rading is high. Can this be lessened by adjusting the length of the antenna?
Yes 2.5 is a bit on the high end. Changing the length of the antenna either longer or shorter by small amounts should lower it. It will take a bit of experimenting obviously until you get it as low as it will go.
2.5 is high enough to not only cause interference issues, but also cause that transmitter to generate a bunch of spectrum junk considering its only got a 3 pole filter. Get the SWR down to 1.5 or less and install the extra filtering and loading on the modules (see priv msg) and retune and recheck VSWR again...peak antenna as needed using "trick" per msg.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
OK, I'm stoked - followed your suggestions re: the antenna and sourced a j-pole from a fellow in wisconsin - for under $75 bucks here it is - and as he tuned it for me to my frequency, the SWR dropped to 1.2 without touching the board. Next is cleaning things up
Nice! However I would put something a bit stronger than those hose clamps for mounting. Use a pair of U-Bolt clamps. Or drill a couple of holes through both the bottom of the J-Pole and the mast and slip the J-Pole inside the mast, run a couple of bolts through the holes and tighten down.
You would be surprised at how much wind loading that J-Pole will have on the mast and those thin hose clamps...even in a light breeze.
Looking good though!
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
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