Hello all,
Hello all,
My AM operation uses a SSTRAN AMT-3000 feeding an antenna suspended from the rafters in my basement and covers my modest estate quite well. Previous range checks while driving have given about 1000 feet on a car radio as gauged by a listenable signal.
But, this week I had a chance to wander onto my neighbor's property for a range check. They were gone and the newspapers were accumulating in their drive so I thought it OK to move them out of sight. I was listening to my web streaming programming and took my portable radio with me so as not to miss a syllable.
At a distance of 180 feet, the audio from my station was not listenable and I could hardly tell there was a signal. My portable is not too shabby in terms of sensitivity, it can pull in a listenable signal from a 5 kW omni station 90 miles away.
The revelation here is the vast difference in range between my car radio and my portable. It raises two issues: First, range is greatly dependent on the receiver used (no surprise) so range claims need to be viewed with regard to the receiver, and second, if one is intending to community cast it is not likely that the intended audience will have a receiver situation any better than my portable which was in the clear away from noise sources. Better stated, it appears based on this simple and perhaps scientifically inadequate experiment that car radio range checks may vastly overestimate the usable range. Anyone else have experience with this?
Neil
Not suprising that there is a great variance in receivers and antennas for AM or FM. My car that has a traditional whip does the best,while the wife's mini-van with the windshield antenna is the worst! A while back I measured the sensitivity of a bunch of my receivers and the results were unbelieveable. The best was about 1uv for a listenable signal to 22mv for another! The selectivity plays a role there too with the congested bands along the great lakes. A better antenna always helps a lot too. Still wish my am would go more than a block 🙁
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Lee,
Thanks for the response. I have a question for you. My receiver uses an internal ferrite loop antenna. Did any of the receivers you tested have this antenna system and if so how does one measure the sensitivity?
Thanks in advance,
Neil
Well mine had loopsticks and an antenna connection point,typical jap stereos. I'm not sure how to go about testing a loopstick only radio. I would imagine coupling it in with a wire loop would be the way to go,but calibrating it could be a problem. Some use an extra winding on the loopstick to run an external antenna,maybe you could add one to yours?I also noticed when I re-tuned these receivers for optimum performance on the loop,the antenna input suffered and vice-versa,so I compromised them to be about equal.Totally unscientific,but they hear better. Some car radios have an antenna trimmer adjustment that can help if tuned right. I also tuned up the FM tuners as well.
My AM transmitter is useless at night from the Mexican flamethrower on 1610,might need to move elsewhere. Nobody listening anyways.
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Lee,
Your results from your sensitivity tests are interesting. I haven't done much for AM but there is a nice way to align and test FM which is called SINAD. I modified a commercial receiver and fine tuned it and achieved a 12 db SINAD of .1 uV which is pretty "hot" for a VHF receiver.
Your suggestion of a coupling loop is good and as you pointed out calibration is difficult. As I mentioned, my receiver hears a 5 kW omnidirectional station 90 miles away so I don't think it is on the low end of sensitivity despite its $15 cost. The thing that struck me was the vast difference in range using this compared to a car radio.
Good point about the car radio "peaking" cap. In the old days it was either behind one of the front panel knobs or on the chassis near the antenna connection. I don't know if or where this is on modern radios but it sure made a difference when properly adjusted on the high end of the band.
Neil
Neil,
About the only way to test the sensitivity of an AM receiver with a loop antenna is to supply a known field strength from a test antenna connected to a signal generator. Measurements can be done in the near field of the test antenna, which may be a small inductor. Near field measurements make it possible to do the measurement in a lab rather than in an open field. It is usually not practical for hobbyists to do these kinds of measurements. The measured sensitivity would not be in microvolts, but in microvolts per meter.
If you have access to Terman's Radio Engineers' Handbook (1943), the problem is addressed on page 973. Terman refers to a detailed 1941 article in Proc. I.R.E. about testing receivers with loop antennas.
Ermi,
Your points are well taken. I don't intend to pursue it, was just curious.
I can see how the sensitivity would be expressable in uV/m in the case of the loop antenna and for those wondering where I got the uV figure, this is for a receiver with an antenna input connector.
Neil
Well,for loopstick radios,use the old ham method of tuning it for max smoke!,,ie,best signal. And wow! a.1uv receiver is hot! I would worry it was too hot and picked up a lot of noise on AM,hopefully it has an rf gain or attenuator control. You can sometimes improve the signal to noise ratio by reducing the sensitivity. I am in the city and have a pretty high noise floor on all bands. Flourescent lights(all of mine are),light dimmers,cable leakage,hi power paging transmitters,errant oscillating mast mounted preamps,computer monitors and tv's all make the bands noisy here...
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
You can add a passive "H Field Multiplier" antenna to vastly improve the sensitivity of any portable radio. You just need a large box, such as a copier paper box. Wind about 20 turns of insulated hookup wire around the box (the long way) and attach the free ends to a 365 pF variable capacitor salvaged from an old BC set. Tune for maximum sensitivity with the radio in the nearfield of the box (it's best to align the windings on the box with those on the internal ferrite bar antenna). You'll be shocked at the improvement! If the capacitor doesn't tune to your frequency, add or remove a turn or two.
However, if you use a GE Superadio III, you might not see much of an improvement, since those are already extremely sensitive. In fact, anyone complaining that they can't hear their station on a portable should get one-- it's the cheapest and most sensitive portable radio you can probably find.
I don't find many car radios with good sensitivity above 1600. You might be able to fine-tune the antenna circuit if you can find an adjustment that you can access.
Oh, and one more thing-- the "Mexican flamethrower" on 1610 is not in Mexico-- it's in Toronto! Check the FCC Media Bureau records for CHHA. It just went on the air this summer. It's only 1 kW at night but since there are no other stations on 1610 except the 10 watters, it gets out like a bandit. Remember when WJDM on 1660 was the ONLY station on X band? They got reports from everywhere in the U.S. including the west coast running 1 kW. Then all the QRM came on...
Personally, I think the AM band is getting worse by the day, with all of the IBOC noise added to the already horrid QRM. Where I live, pretty much the entire band sounds like $#&! at night. Thanks to the FCC for allowing more and more stations at higher power levels into the band... and all of the consulting engineers for doing the move-ins for the big media conglomerates...
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