[aprssig] 9600 Baud East Coast Backbone (15 radios?)
bruninga at usna.edu
Sat Aug 20 17:54:53 CDT 2016
> The other issue is that these are voice radios.
> You won't be able to directly tap the discriminator or TX modulator
> for optimal "flat" TX/RX audio response for... 9600 baud.
You nailed it. OK, back to square one.
Looking for 15 identical radios cheap with discriminator and modulator
access for 9600 baud backbone. And easy to program. 2m or 220, or 440 MHz.
On Fri, Aug 19, 2016 at 11:28 PM, Stephen H. Smith <wa8lmf2 at aol.com> wrote:
> On 8/18/2016 4:52 PM, Robert Bruninga wrote:
>> 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
>> 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.
> 2 DB loss ???? That's horrible, though it may be due to the lossy
> miniature coax.
> 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
>> worst, RG-174 simply because it was there. Im wondering what I can get
>> higher quality coax. The length of the open stub was about 12.6".
> More serious is that the stub is a narrow-band device. It can't really
> do anything about multiple transmitters spread out across a band, let alone
> FOUR bands ( 30-50 MHz VHF-LO, 137-174 MHz VHF-HI, 410-508 UHF+"T-band"
> and 770-900 MHZ formerly called "800 MHz" where numerous trunked systems
> using 5-10-20 channels each exist. (You WILL get a mix of all these
> bands on any busy shared mountain-top site.)
> [I couldn't even get a simple stub to remove all of a single 6-MHz-wide
> VHF-HI TV channel.]
> The typical band-pass filter used in commercial sites for receivers will
> consist of half-a-dozen helical resonators in a metal casting of multiple
> small cylinders. The individual series LC circuits are aperture-coupled by
> small slots in the walls of adjacent cylinders. These yield the
> characteristics of classic over-coupled IF transformers or SSB
> crystal-lattice filters; i.e. a flat nose with steep skirts. Depending on
> how the individual resonators are stagger-tuned, the nose bandwidth can be
> anything from .5 MHz or so to 10-20 MHz.
>> So my question was more along the lines of how much out-of-band isolation
>> needed and then see what kind of filter we can use.
> Again, unless you have specifically-tailored BANDPASS filters that "drop
> dead" outside of a one or two MHz bandwidth, you will have signals on
> multiple bands/frequencies coming down the coax, overloading and mixing in
> the radio's front end and diode T/R switch,then traveling back up the coax
> and re-radiating.
> And I am talking about
>> having the HT inside an RF box along with all the filters.. just to see
>> is possible.
>> I think I'll order a pair of radios to do some testing.
>> Bob, WB4aPR
> The other issue is that these are voice radios. You won't be able to
> directly tap the discriminator or TX modulator for optimal "flat" TX/RX
> audio response for packet, let alone the DC coupling for 9600 baud. You
> will only have access to the speaker & mic jack. To emulate the flat
> response of the Kenwood radios at 1200, you will need to place a
> DE-emphasis network externally on the the TX side and a PRE-emphasis
> network on the RX audio out to cancel out the internal PRE/DE emphasis.
> Which still won't address the DC coupling needed for 9600 TX and RX.
> [I actually dealt with this issue over 3 decades ago. I was a systems &
> field engineer on one of the first public-safety mobile data systems ever,
> for the City of Los Angeles PD. We had to mate the mobile data terminal
> (which used a 5" B&W CRT and a box with over 300 small-scale CMOS and TTL
> chips) to the city's existing VHF Motorola Motrac and Micor radios in 500
> police cars. We wound up designing and installing 500 copies of a TX/RX
> audio equalizer box that inserted between the radio and the terminal's
> modem to cancel the uneven audio response of the Moto TX and RX audio.
> A decade later, I went through basically the same drill with the County of
> LA Sheriff's fleet of 600 UHF radios. An added twist here was that each
> car had TWO UHF radios in the trunk - one for voice and one for data. We
> used retuned compact UHF duplexers, from the old pre-cellular IMTS mobile
> phones, to isolate the voice and data radios in the same car.]
> There may not even be a discriminator to tap in this handheld, even if you
> ARE willing to hack a minuscule PC board under a stereo microscope! An
> increasing number of these low-cost Chinese handhelds are based on cheap
> cellphone radio chip sets that use direct conversion and DSP rather than
> classic RF/IF/discriminator chains. All the modulation and demodulation is
> done with smoke, mirrors and software......
> 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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