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[aprssig] HF options

Stephen H. Smith wa8lmf2 at aol.com
Wed Dec 19 19:23:31 UTC 2007


Scott Miller wrote:
>> NVIS is a relatively "lossy" mode meaning you have to run fair 
>> amounts of power to achieve a reasonable S/N and data error rate. 
>> (The lower HF 
>
> Yeah, I figured NVIS wouldn't be great... I was hoping there was 
> something with adequate ground wave propagation to be useful.
>

If you really want to do groundwave rather than NVIS skywave, you could 
make like commercial AM broadcasters - tall vertical ants over serious 
radial systems (i.e. 10-20 quarter-wave radials in or on the ground).   
A full-sized quarter-wave vertical on 3.5 MHz would be 66 feet, although 
one could load it to perhaps half that without giving up too much 
efficiency.  [One appoach, since the vast majority of the radiation is 
from the first 1/3rd of the quarterwave ant,  is to go UP 33 feet with a 
pushup mast,  and then out horizontally for the remaining 33 feet with a 
piece of wire. Again the thing needs to be over a serious ground plane 
mat.] 

Such a setup should be good for a groundwave range of 30-40 miles, 
assuming the local ambient noise level wasn't too high.


>>
>
> I figured a crystal-controlled rig would be better for simplicity.  
> But you're right, doing data at low S/N isn't going to be fun, and 
> AFSK over SSB introduces frequency stability concerns that you don't 
> have with FM.

Frequency stability isn't an issue with modern synthesized HF 
transceivers, especially on the lower HF bands, where the 
parts-per-million stability issues amount to only 5-10 Hz absolute long 
term.    [I have a Kenwood TS-50 running my 30M HF igate that has stayed 
within +/- 3 Hz for over three years 24/7 on 30M. without the optional 
high-stab oscillator.  It helps that it is in the civilized 
constant-temp environment of my living room rather than being 
alternately baked and/or frozen in a mobile.]

>  If the remote stations are transmit-only, though, it seems to me that 
> you'd only need a small number of more capable RX stations.
>

My first choice for this kind of application (assuming you are not 
talking fixed channelized commercial HF rigs) would be the Elecraft K2 
transceiver.  The basic radio is a 10W output synthesized transceiver 
meant for battery-powered QRP operation.  An optional internally 
-mounted PA can boost the TX output to the customary 100W. 

The RX has VERY low power consumption (under 100mA at 12VDC with the 
panel lights off), and has extremely high dynamic range with a nearly 
"uncrunchable" front end, comparable to serious "competition-grade" 
high-end radios.   [On the lower HF bands, enormously strong off-channel 
signals, especially at night, that block or desensitize receiver front 
ends are always a problem.  Super strong signal handling on the lower 
bands is always far more important than sheer receiver sensitivity since 
ambient noise limits the weakest signal you can hear. ] 

Part of the reason for the excellent RX specs on this relatively 
inexpensive transceiver is that it is NOT continuous coverage like most 
modern radios. It is ham-bands-only with band-pass front-end filters, 
instead of the wide-open front-end RF stages that suck in practically 
everything from DC to light, present on most contemporary radios.

Further, the receiver of this radio uses classic passive high-quality 
crystal IF filters with good group delay characteristics for data 
operation.



--

Stephen H. Smith    wa8lmf (at) aol.com
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