[aprssig] 9600 Baud East Coast Backbone (220 stub filter?)

Robert Bruninga bruninga at usna.edu
Thu Aug 18 16:52:12 EDT 2016

Agree, the radio cannot go in naked.  My question was more along the lines
of how much filtering is needed since the 150-170 MHz bands and 450-470 MHz
bands are quite far from 220.

I just did a quick 1/4 wave coax stub that notches 153 and 460 MHz with 30
dB notches and has only 2 dB insertion loss at 220.  I'm thinking that with
a multistub approach, it might be possible to get quite a bit of filtering
and lower insertion loss with just some coax stubs. The coax I used was the
worst, RG-174 simply because it was there.  Im wondering what I can get with
higher quality coax.  The length of the open stub was about 12.6"

So my question was more along the lines of how much out-of-band isolation is
needed and then see what kind of filter we can use.  And I am talking about
having the HT inside an RF box along with all the filters.. just to see what
is possible.

I think I'll order a pair of radios to do some testing.
Bob, WB4aPR

-----Original Message-----
From: Stephen H. Smith [mailto:wa8lmf2 at aol.com]
Sent: Thursday, August 18, 2016 1:17 PM
To: Robert Bruninga; TAPR APRS Mailing List
Subject: Re: [aprssig] 9600 Baud East Coast Backbone

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'

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
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

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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