Reducing Fading Distortion

[flintstone mop]

Hello All,
I read somewhere that one way to reduce the distortion from selective fading was for the broadcaster to reduce the modulation of the Transmitter to 30%. True or False?
I wish I had the coins to buy a nice sounding SW radio with a real Synchronous Detector. I have also heard this is THE best way to reduce distortion.
I have noticed as I listen to 'BCQ that I will connect the monitoring receiver to various aerials and they will produce various degrees of fading distortion. My best antenna for the least distortion is 600 feet of wire laying on the ground, going every which way.

I was tuning around last night on 40 and there were a couple of stations that sounded really good, for transmitting music (7250). The announcer sounded German. It was almost pegging needle on the R390A. I guess if you're pumping out a half million watts you can get away with lowering the modulation. Unbelievable! This was around 10:30PM Eastern
Fred

[WA1GFZ]

I hear a 10 dB  linear helps

[David, K3TUE]

I wish I had the coins to buy a nice sounding SW radio with a real Synchronous Detector. I have also heard this is THE best way to reduce distortion.

A Sync Detector is nice indeed.  The Drake SW-8's, are not too expensive at around $400.  While not hi-fi, they do have a 4 and 6kc filter.  And the sync det, while lacking the selectable sideband of an R-8, was otherwise as good,  (read fabulous).

[Bacon, WA3WDR]

Try receiving with Circular Polarization.  Set it up so you can switch the CP 'sense' right or left, because this changes at different times and over different paths.

Two inverted V antennas at decent height at right angles to each other, with equal length feedlines and connected through a 1/4 wave delay line that can be switched into either feedline for 'sense' switching, makes a decent CP antenna for relatively high angle stuff.  My experience was that TX and RX were optimum with opposite senses, so for transmit and receive, set the phasing switchbox up with a T-R switch (see diagrams).

A couple of ferrite rods antennas at right angles, with preamps and +/- 90 degree phase delay combination would also work, and could be aligned broadside to the incoming wave regardless of incoming angle, although the horizontal component is attenuated near the ground, so it would work better up maybe on the higher floors of a building, or on the roof.  Equal tuning of both loopsticks and equal gain in each preamp are important, as is the 90 degree delay.  'Sense' is determined by which antenna gets the 90 degree delay.  Then tie them together with a T, or mix them with active circuitry with a broadband 90 degree delay, and feed the combination to a receiver.

Probably the best arrangement short of a complex DSP combiner would be a simultaneous active combination producing right-CP, left-CP, and each antenna directly, into a four-input voter receiver.  Whichever one had the best signal at any instant would produce the output.

Most short-term HF fading is due to polarization rotation, which whips around depending on a lot of factors.  When the incoming signal polarization is cross-polarized to your antenna, the signal fades, and weaker skip modes with different path delay become more significant and cause varying delayed recombination, which causes selective fading.  Even the main signal path is broken into an 'ordinary' (stronger) and an 'extraordinary' (weaker) components, and these follow slightly different paths through the ionosphere and have opposite circular senses.   I clearly heard a phasing audio effect removed from receive signals when I used the optimum circular sense. When the 'ordinary ray' fades, the 'extraordinary ray' can cause selective fading.  This gets worse when the band is about to go long, because the stronger ordanary ray fades out first.  The extraordinary ray gets relatively stronger, and it causes more interference, and then it is the only working path, and then it fades out too.  With a CP receive antenna, you will see the optimum CP sense flip just before the band craps out.  At least this has been my experience.

[Pete, WA2CWA]

I think fading is cool. It adds realism to listening to a short wave broadcast.

[K3ZS]

Your 600 feet wire wire in random directions gives you the space and polarization diversity that much more expensive government installations try to achieve.

[W8EJO]

The Redsun RP3000 is due to come out soon and it will have sync., detection, ssb & retail for under $150.

http://www.fenu-radio.ch/Redsun%20RP3000.jpg

[Todd, KA1KAQ]

If you've ever watched the movie 'Where Eagles Dare' with Clint Eastwood and one of Liz Taylor's ex-husbands(x2), the scene of the radio room is one of my favorites. The German op is sitting there listening to accordion or some such music as Clint sneaks in to whack him. The signal rises and falls, exhibiting overloading as well as distortion and fading. I always wondered if it was just a good recording, or if they really had one of those receivers going.

Certainly realistic, to those of us who appreciate such things.

[WA1GFZ]

Buy a $14 softrock kit and interface it to your IF. Then use software sync detector do the work. Software is free. fc

[WU2D]

I think that space or polarization diversity would be easiest to get working.

For 75M a Vertical and a Dipole spaced out a bit should do the trick.

The combining can occur pre-detection or post detection.

Pre-detection is fairly hard to accomplish. For post detection one can simply throw one of the signals (the faded one) away. This is done by observing the AGC of the receiver and using it to switch to the opposite front end any time the RSSI goes below a trigger level.

Actually a pair of 75M front ends sharing the same crystal LO which convert down to a receiver tuned to the broadcast band is a starting point.  Then you connect a comparator to the AGC.  The comparator flips from one front end to the other (each connected to a different antenna). Obviously hysteresis is required.   

This is not combining - it is voting. This system would help with deep fades. Along with synchronous detection which improves the sideband distortion situation, this could result in tremendous multipath rejection.

Mike WU2D   

[WU2D]

Of course this voting scheme would not work with a weak synchronous detector system which is unable to lockup quickly or remain locked.

[WA1GFZ]

Get yourself a cubic 3030 and put 1 RX on the dipole and the other on the vertical. Plug in stereo headphones and let the brain sort it out.

[Blaine N1GTU]

listen to the stations simulcast on the internet... 

[wa1knx]

bacon,
    I remember you using cirular polarization on 75 years back
and you had less fading with the ant, on my rcv side as well!

[Bacon, WA3WDR]

Hi Dino - yes, somebody, I don't know who, told me that it was possible to 'excite' the desired ray (ordinary or extraordinary) by transmitting with the right CP sense.  K1DEU John told me that he heard me with less fading when I transmitted CP too.  Those were interesting tests.

I noticed that the day-night and the night-day transitions did not exhibit a preferred CP sense, and I couldn't find an optimum CP sense during aurora flutter.  But in normal steady conditions, the right sense CP really made things better.  Also the European DX was optimal with the opposite sense than was best for local stuff, so CP gave me a little extra interference rejection when receiving 75 meter DX.  And I have no idea why TX and RX would have opposite preferred CP senses.

[flintstone mop]

The fading does something magical to the station. It's the grinding wrenching distortion of the music that drives me batty. Especially when I want to rock out on 7415 on Thurs. For some reason it more pronounced on 'BCQ's freq. I don't know if it has to do with their location on the Earth and being closer to the Aurora or their transmitting antenna which is an expensive log periodic with a beam heading to the Carribean. The antenna is mounted on a 60 foot tower.
The selective fading is more tolerable on the AM broadcast band than on 40M. Listening over the Internet takes all the magic away.
Thanks for the inpoot. I have printed this thread for the ideas mentioned.

Fred

[Bacon, WA3WDR]

I always noticed that when W2VJZ Irb would make a dramatic statement, he would then pause for about ten seconds of dead air, and there would always be a short, deep fade in the middle of the dead air that kind of underscored his point, something like a judge banging his gavel.

[WB2CAU]

Back in the mid to late 80s when I had my 75m CCD antenna up, I noticed significantly less signal fading on receive with it.  It was very obvious that the CCD had less fading after I replaced it with the 160m dipole and the signal fading was back to "normal" again. By the way, the signal fading was also reduced on my transmitted signal when using the CCD.

Eric - WB2CAU