Midwest and FM DX'g...
> Their isnt that much noticable difference between 8 DBF, and
> 10 DBF.. Although, the lower the number, the better the
> reception. If Carls station is a part 15 fm, it probably
> covers about a quarter mile, depending on how high his
> Antenna is mounted. I can pick up my part 15 fm Between a
> Quarter mile, to a half mile, but I have detected my signal
> a full mile away at times, This is with (NO) outdoor
> antenna, with the TX at ground level, with a Wire taped to
> my window, However, Yesterday, a huge DX opening was
> preventing me from barely transmitting around the block, so
> alot of it depends on how clear Carls frequency is, part 15
> just isnt powerful enough to overide Licensed stations, when
> they come Booming in from a distance.
>
Midwest,
I'm sure you're into FM DX'g from the sounds of the problem you have below, but you can improve on your distance and sensitivity, but alas the transmitter ouput is the biggest challenge, below I've posted some of my results with this:
Fm has two types of listeners, one is the kind that listens to stations close to their city or town of reception and is for entertainment purposes only, the second kind is the long distance listener that wants to pick signals on FM as far away as possible (DX), these stations are as far away as 1000 miles away or farther in some cases. Listening to these particular stations one must have an excellent tuner with sensitivity and selectivity for recieving and capturing such weak signals.
There are only a few excellent models of FM tuners available for the purpose of long distance FM reception (FM DX) at
present and most of these are very hard to come by either though Ebay auctions, audio Hi-FI warehouses, and or electronics stores.
Today, most audio hi-fi stores mostly sell AM/FM receivers (tuner and amplifier in one unit). Receivers are quite different from dedicated FM tuners, in terms of selectivity, sensitivity, and image rejection. Even just an FM tuner with no AM or amplifiers is better than the complete AM/FM Stereo combo in terms of quality components and IF stages, etc.
Radioshack in it's earlier days had very good FM tuners but are mostly gone or lost through time and neglect, the new stuff is junk. I still have the 'Realistic FM Tuner Model TM 1000' bought back in 1978 and it is very well made, not the cheap stuff made today... the tuning capacitor is approx 2x3.5 inches alone and the IF stages are excellent with very good sensitivity and selectivity.
Back in 1995 when I moved to Savannah, I had to learn how to correctly interpret FM tuner specifications to get the feel for finding that perfect tuner/reciever to get stations in the Caribbean and South Florida locations, so I ended up buying a model that didn't quite not meet the specs as needed but good enough for a start in modifying it to get close to the untouchable 'Onkyo T series of FM tuners', the 'Onkyo T-9090 II' being the 'primo' in terms of sensitivity and selectivity.
Even some portable FM/AM/SSB radios from Grundig (Satellit 700) and the Panasonic RF-B Series radios with just whip antennas get reception of signals as far away as 900 miles.
The Grundig S350 does a good job on the FM side if it's hook up to my FM Yagi... JasonW, hope you got yours and testing it to see how you like it.
Right now these are the 'best' FM tuners you can get your hands on in terms of FM long distance recieving (DX), Ebay is a good choice to try to find these and audio/electronics repair or salvage is another.
Onkyo T-9090 II
Onkyo T-4711.
Onkyo T-488F.
Denon TU 1500RD
Yamaha T-85.
Sony ST-SA5ES.
Kenwood KT-6040
Me, I have the Sony STR-D615 FM/AM Tuner and it's modified at the IF stages to improve signal/noise ratio and improve the sensitivity/selectivity. This receiver is coupled to an FM 8 eight element yagi antenna, which has approximately 6 dBd gain according to the manufacturer... it could be improved by adding more elements. I've been able to get stations as far away 300-1000 miles away, maybe not full stereo and of excellent listening quality but listenable, sorta like shortwave DX'g.
Some of the sources here are borrowed from the FM DX'er sites saved for reference:
Sensitivity:
Sensitivity at FM frequencies of 88-108MHz makes FM DX'g, better suited to hearing better quality signals
(atmospheric and man-made noise is alot lower than AM). The surrounding noise levels at 88-108 MHz are
typically 2-5dB, so the FM tuner's RF amplifier front-end noise is very important to be the lowest possible.
FM tuners that are very sensitive usually feature Mosfet transistors in the first RF (radio frequency) stage.
The low noise features of mosfet transistors makes them very desireable, sensitivity for weak signals is
excellent when used in the front end of the IF stages.
Noise figures are about 2-3 dB for quality sensitive FM tuners that employ MosFETs in the first RF stage.
Modern FM tuners (post 1987) are capable of producing extremely weak reception you can just hear with signal inputs as low as 1/10th of a microvolt (0.1 uV). At 1 microvolt (1 uV), a quality tuner will produce enough FM quieting to enable almost noise-free reception.
How do you know if your tuner is suitable for FM DX'g?
Most sensitive FM tuners are able to detect FM signal meteor scatter "pings" almost anytime of the day. If meteor scatter pings can't be heard on vacant channels, the tuner's RF sensitivity is not good enough for FM DX, The tuner must be connected to an efficient high gain external directional high gain FM antenna such as a FM yagi with sufficient elements.
Sensitivity ratings are usually expressed in microvolts (uV), and the lower the number the better. Another rating scale sometimes used is the femtowatt (dBf). This indicates a tuner's sensitivity, regardless of its input impedance (usually 75 or 300ohms). In most tuner brochures, you will find the IHF sensitivity rating, which is for weak mono signals. This corresponds to a -30-dB level of total harmonic distortion plus noise (THD+N), that is a mono signal with 3% distortion. The IHF mono sensitivity specification is also called useable sensitivity, which means the weakest signal most non-DXers would be prepared to tolerate. IHF useable sensitivity usually ranges from 0.6uV (6.7dBf) to 1uV (11.2dBf). The lower the figure the better.
A tuner's ability to receive weak stereo signals is indicated by the IHF usable stereo sensitivity figure, which usually ranges from 2uV (11.2dBf) to around 4uV (17dBf).
The 50dB quieting level will indicate how strong a DX signal has to be to reach hiss-free level, and that is often more important to those who like HI-FI FM DX. At 50dB stereo signal-to-noise-ratio, the tuner's hiss is only 0.3% of the audio, and in a good FM tuner, that will be obtained with no more than 17uV (36.1dBf). The 50dB mono quieting specification is usually no more than 1.7uV with a good tuner.
By using a low noise (2dB noise figure) tunable MosFET FM preamplifier, it's possible to achieve state-of-the-art RF sensitivity which will beat any top-line FM tuners. Even if the pre-amplifier is used indoors, the tuner's RF front-end noise figure is improved.
If you find that the sensitivity of your FM tuner is not improved by the use of a pre-amplifier, the external noise levels are either fairly high, or your FM tuner's noise level is sufficiently low, so then external noise becomes the limiting factor.
Selectivity:
Selectivity refers to a tuner's ability to receive a weak station in the presence of stronger stations at slightly lower or higher frequencies.
"Alternate" selectivity refers to the tuner's ability to suppress interference from signals plus and minus 400kHz away from the tuned frequency. A rating of 80dB is good; while 100dB is excellent.
"Adjacent" selectivity refers to suppression of signals plus and minus 200kHz away from the tuned frequency, and is usually no better than 50dB on a FM tuner featuring narrow IF bandwidth.
One example of a tuner with good selectivity is the Onkyo T-9090 II FM tuner, which will suppress signals plus and minus 300kHz away from the desired station by a factor of 10,000 (80dB). Signals plus and minus 400kHz away from the desired station are suppressed by a factor of 56,200 (95dB). These figures indicate excellent shape factor selectivity, considering the -3dB point is 150kHz.
The unmodified Sony STR D615 AM/FM Tuner specs are (60db) at 400khz with a 4.5uv of sensitivity at 50db quieting and 2uv 11.5dbf sensitivity.
Modifying the reciever for high selectivity is achieved by using several 150kHz ceramic filters in series in the 10.7 MHz IF strip. Some DX enthusiasts have replaced the stock filters with narrower MURATA 110kHz filters (part no: SFE10.7MHY-A).
Recommended 10.7 MHz IF filters for enhanced FM tuner selecivity performance.
My Sony STR-D615 has selectivity of plus 75db fitted with the Murata SFE10.7 IF filters.
Certain ceramic IF filters are designed for high selectivity and fidelity. The Murata SFE10.7MZ2 is such a filter.
Providing your FM tuner has sufficient IF gain, five 110 KHz filters connected in series, gives the optimum selectivity performance. Five 150 KHz filters connected in series gives the optimum selectivity / fidelity performance.
FM tuners featuring switchable wide or narrow IF bandwidth will have superior selectivity. This is a highly desirable feature for successful FM DXing.
Strong signal handling and spurious rejection:
Many cheaper FM tuners will suffer from overload and spurious images of local stations all over the dial, particularly in city areas. This is often because the tuner's RF amplifier is a bipolar transistor type and has no pre-selection. To achieve overload immunity, a tuner should have a MOSFET RF amplifier front end, tuned with varicap diodes. Dual gate MOSFETs are excellent because they operate linearly over a wide range of signal strengths and also offer a low noise figure, hence feature good sensitivity.
Conclusion is that obtaining the proper equipment and understanding why one can get extremely distant FM stations are just a matter of understanding the proper terms in the specifications of equipment and being able to tweek and modify as needed.
Radiopilot
