[GreenKeys] Space diversity receive antenna

[Harold Hallikainen]

Thanks! I currently have a 30 foot vertical. I am thinking of adding a 20
meter horizontal dipole where the closest end is about 30 feet from the
vertical. When the receive signal fades below a threshold (perhaps the
mark hold threshold), a relay would start going back and forth between the
two antennas until we get a signal above the threshold.

On frequency diversity, as shown at
https://w6iwi.org/script/csvgraph.html?CsvUrl=https%3A%2F%2Fw6iwi.org%2Frtty%2FTuNotes%2Fms220517a.csv
, there is some time offset in the fades of mark and space with 170 Hz
shift, but not enough to keep one of them out of the noise. I need to run
the experiment with 850 Hz shift.

Time diversity, typically repeating the message with a way to detect
errors, could be used, but requires special equipment so we don't have
each line of text printed multiple times. By the way, I designed this
cinema closed captioning system (
https://www.movingimagetech.com/images/products/mit-ADA-Accessibility/R000301_IRC_Manual_v23-08-01.pdf
). The system transmits two channels of audio plus closed captioning over
IR. The audio channels are FM at 2.3 and 2.8 MHz. The closed captioning
data is 10 kpbs FSK at 1.8 MHz. The data is packetized and includes a CRC
on each packet. A caption is transmitted continuously from the end of the
previous caption to the end of the current caption so the receiver has
multiple times to receive it without error (as determined by the CRC).
So... there's my time diversity system!

Thanks for the comments! I will try to get the dipole up in the next few
weeks. It will provide both space and polarization diversity. It will be
interesting to see if it improves performance.

Harold
https://w6iwi.org


On Sun, January 5, 2025 2:40 pm, Brooke Clarke via GreenKeys wrote:
> Hi Harold:
>
> Yes, diversity is a very good idea on HF RTTY and was the standard for a
> long time.
> Note there are different flavors of diversity, for example two identical
> antenna systems spaces a wavelength or more
> apart gives you space diversity.  There's also polarization diversity and
> frequency diversity.  Minimum Shift Keying
> (MSK) has little frequency diversity, but there's some for shifts that are
> wider than the minimum.  An extreme case of
> frequency diversity is "Chirpcom" where a 40 character message is sent
> over and over in time while the carrier sweeps
> from 2 to 30 Mhz.
> https://prc68.com/I/RCS-5A.shtml#Cc
>
> Time diversity also works.
> The MD-1142/UGC is a time diversity modem.
> https://prc68.com/I/MD1142.shtml
>
> --
> Have Fun,
>
> Brooke Clarke
> https://www.PRC68.com
> axioms:
> 1. The extent to which you can fix or improve something will be limited by
> how well you understand how it works.
> 2. Everybody, with no exceptions, holds false beliefs.
>