[aprssig] 9600 Baud East Coast Backbone

Stephen H. Smith wa8lmf2 at aol.com
Thu Aug 18 13:17:16 EDT 2016

On 8/17/2016 11:34 PM, Robert Bruninga via aprssig wrote:

> I wonder if the 5W 220 MHz HT featured in QST this month would make a possible
> radio.  Many of these sites are high and adjacent to plenty of VHF and UHF
> commercial rigs.
> Does that mean the 220 MHz is relatively free of front end overload at most
> commercial sites?
> Could just a simple 1/4 wave coax stub bring the RF levels down to workable levels?
> The radios are 5W and show 0.16uv sensitivity for $85.  Moving the backbone to
> 220 coiuld then allow either 2m or UHF for local user access to the backbone.
> I had wanted 50W rigs for the 10 dB margin, but maybe 5W will do some of the
> links.

Putting a cheap crap handheld like this on a mountain-top adjacent to existing 
crowded sites is appalling!

Due to space constraints, handhelds don't have much in the way of RF 
selectivity in their front ends.  Further, they normally have diode T/R 
switching rather than mechanical (metallic contact) antenna relays.

If you are lucky, -YOUR- receiver won't be desensitized by hundreds or even 
thousands of watts of RF from other transmitters on every frequency from DC to 
light. (Most likely it will).

However, unless you have a string of high-Q cavity filters between this radio 
and whatever antenna you connect to it, it is virtually GUARANTEED to become a 
broadband interference and intermod generator to OTHER systems' receivers.

10'S OF VOLTS of RF on every frequency will come down the coax from the 
antenna, mix in the radio's overloaded front end or diode T/R switch, and then 
travel back up the coax and be re-radiated.   The result is that you will be 
unwittingly generating massive amounts of intermodulation interference to OTHER 
users on the site.

[The majority of non-ham transmissions are now digital in one form or another 
(i.e. sounds like white noise or a grinding roar in analog receivers). Many 
shared or trunked systems have full-duplex control channel radios that transmit 
continuously 24/7 with a hundred watts ERP or more. As a result, a lot of this 
interference will manifest itself as an elevated white-noise level for other 
users receivers, rather than the traditional burps, squawks, squeals and babble 
of voices that "intermod" has been in the past.]

Diode mixing in the output stage of solid-state transmitters of other 
transmitters' outputs is not  uncommon,  which is why virtually all responsible 
site managers at crowded commercial sites now MANDATE high-Q filters and 
circulators/isolators for every radio on a site.

As a result, typical commercial land-mobile & public safety radios have 
separate transmitters and receivers so that bandpass filters/splitters can be 
placed in front of multiple receiver front ends, while separately 
circulators/bandpass filters/combiners can be placed in front of multiple 
transmit outputs.


Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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