What I mean is, that this is a continuation of rock95seven’s “Scosche FMT-4 Review” blog, which is well worth browsing through so that what I say now makes more sense.
What I mean is, that this is a continuation of rock95seven’s “Scosche FMT-4 Review” blog, which is well worth browsing through so that what I say now makes more sense.
rock95seven’s discovery of a $10 CERTIFIED FM transmitter got me back into FM, which I left when I sold my Ramsey 25B. Now what about that. After building the 25B I tinkered with it for awhile, decided to concentrate on AM, and sold the 25B to a friend. Is that illegal? Never mind. That’s not why I’m calling.
Another outstanding feature I’ve noticed about the Scosche, is that the audio hasn’t overmodulated, even though I have no limiter attached. This suggests an in-built limiter, and it seems to be unnoticable. Tonight I’ll really overdrive it and see if I know what I’m talking about.
Scosche isn’t in my dictionary, but I would have sworn that it’s a vocabulary word. It would be used like this: “How much vodka do you want in your drink?” Answer, “Just a scosche.”
The amazing ranges achieved with various antenna lengths are reported in the other blog, but this addendum adds more to the story. When tested the Scosche was plugged to my indoor audio system, which adds a major layer of grounding. But today I tried a reverse setup.
I wanted to fine-tune the Ramsey AM 25 at 1550 KHz with its indoor antenna. So I took the C.Crane Plus radio back to the mulch pile about 100′ behind the house where the reception was very wobbly. I plugged the Scosche to the radio’s audio out and put the AM 1550 signal over to 107.1 MHz FM and went back indoors.
Reception of FM 107.1 was terrible even using a 1/4-wave antenna which had done a solid 400′ from inside the house. I raised the whip on the RCA Super receiver and added two more feet with clip-lead so that I could indeed diddle until the AM 1550 fringe signal was as buff as I could get it.
Conclusion? Probably that the grounding matters a lot for distance transmission on FM.
At the same time I scrolled through forums and blogs reading FM transmitter reviews and could now hypnotize a dinner companion until she nodded face-forward into her soup while I went on about FCC certification and how milliwatts count under the law.
As soon as I hang up I’m getting a C.Crane FM transmitter so there’ll be a point of reference in this madness.
C. Crane FMT FM Transmitter
On previous threads I recall that no link may exist to the FCC certifying information on the C. Crane FMT certified low power FM transmitter. Here is that information:
https://fjallfoss.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=502193&fcc_id=%27BYG006%27
Checking Those Harmonics
It may be true that perhaps a main reason for restricting the power of Part 15 on the FM band has more to do with its harmonic frequencies than with the primary signals. I decided to give serious attention to the harmonics, and right now here is what is known.
The second harmonics from the FM band all fall within the VHF High Band, which was analog TV channels 7 – 13, frequency band 174 -216 MHz. Some radios have TV band and would allow checking these frequencies.
The FM band third harmonics all fall on government and military channels, 335 – 450 MHz. At this time I am not sure how to look for harmonics up there.
So far as I can tell the FM band has no harmonics on aviation radio frequencies, but I recall hearing anecdotes about pirate FM interfering with flight channels. Is there any truth to that?
Asked and Answered
Universal Radio has radios that tune the gov/mil frequency band.
FM Harmonics and Spurs
Your comments about the harmonics are well taken but I want to mention that a more serious problem can exist in the form of spurious emissions (spurs). These can appear at any frequency including the aircraft and public service bands and generally exceed the power levels of harmonics.
Though a certified unit should not produce harmful spurs it is possible that modifications, especially to the power supply, can introduce this problem.
For example, in my early ham radio days I used an amplifier to kick my signal from my hand held while mobile up from 2.5 watts to 25 watts. I received a call from a police department which heard my communication on their 156 MHz system even though I was using the 146 MHz band. They tracked me by my ham call. They were very nice about it and I apologized and ceased operation. This definitely was not a harmonic problem. A bench test revealed that the power supply to the amplifier was not properly bypassed. I fixed that, checked the unit again, and went back on the air with no problems. I wonder if I had been using a transmitter without a license if the police would have been so cordial.
Neil
Spurs Of Course
I remember the FM engineers talking about spurs but it didn’t click until now what that meant. Neil, if you weren’t on part15.us then part15.us wouldn’t be part15.us
Oh, and Universal has radios that will cover all the bands, including aviation.
Now the matter of the C. Crane FMT certified transmitter, which arrived today, and is now running at 107.1 MHz. I knew it would be weak, but it’s far weaker than I could have imagined. Of course there are trials and errors ahead with other placements and ground planes, but for now I get a good signal out to 6-feet. IT DOESN’T EVEN FILL THE ROOM. Of course I’ll be communicating with C. Crane.
The mystery is that some reviews reported much better results.
CCrane FM
Carl,
Your compliment is appreciated but all of us make this board what it is.
You should definitely have a range of over six feet with this unit. I have every confidence that you will find out what is happening and fix it and report back. (BTW maybe start by taking the screens off of your windows :-)).
Neil
The C.Crane Report # 2
In the first test the C. Crane FM transmitter Model No: FT-007 was atop a five foot stack topped off with a metal enclosed computer and a Linksys Router. The router is not connected to anything. Atop the router the transmitter was 7″ from the nearest wall, and 10″ from another wall, this being a corner. The effective signal range using a C.Crane Radio Plus amounted to about 6-feet. In other rooms there were scant traces of signal.
The second test, now completed, featured a stack of plastic filing cases reaching four feet off the floor, with two aluminum license plates serving as a ground plane with the transmitter in the center. Distance from the nearest wall was about 2 1/2-feet. Much better results with the transmitter fairly achieving the advertised ability to be receivable throughout the house. Some locations are touchier than others, but we are now convinced that standing away from obstructions, raised high and ground plane added, the FT-007 can be optimized.
By comparison, The Scosche FMT4, previously tested, is evidently stronger and therefore performs well in odd locations.
C.Crane Report # 3
The following information is entrusted to us because we are responsible, reasonable, thoughtful and always do the right thing.
http://www.outlawradio.us/CCrane_Mod.html
Telescoping Whip
I think what i will try is finding a tall plastic spaghetti container and see if i can connect a telescopic whip antenna to the FMT 4.
Drill a hole in the side of the container for the audio/power wires and place everything but the power source into the container with the solar cells i should be receiving this week or next, mounted on the side of the container.
Mount the contained FMT 4 on a pole outside on the porch and see what kind of coverage I get. The telescopic whip will be adjustable so I can limit the range that way.
On second thought the antenna will probably end up on the outside of the container. 🙂 Might even be able to add three elements on the bottom of the antenna for a ground plane.
My ramblings for a Monday Evening.