Re: Design for Gunnplexer system


Richard Bown <richard@...>
 

On Sun, 14 Apr 2013 11:53:07 +0100
David Kirkby <david.kirkby@onetel.net> wrote:

What do people think about using this as the 88-108 Mhz receiver kit, based
on the TDA7000,

http://www.ebay.co.uk/itm/140600132991
and the accompanying audio amplifier
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130571695212

with a view to later changing to a 30 MHz IF by returning it to 30 MHz?


Another option, which is even simpler, is this little kit, which has the
audio amp on the same circuit like Peters. It uses a varactor diode for
tuning.

http://www.ebay.com/itm/TDA-7000-FM-RADIO-RECEIVER-88-108MHz-DIY-KIT-/261159434791
That kit has a free running vco, so dont expect it to be very stable, you are much better off using
one of the cheap vhf/uhf handhelds and using the wide band receive function , at least its
synthesised, and more stable, and a lot more sensitive, depending on which HT you use most will
tune WBFM down to 30 MHz so 70 MHz wont be a problem. Those HTs can be got for about £30 ~£40
and they work and do not need building, so you only have the down converter to build. I expect most
of your club members already have these handhelds, or have a kenwood TH-F7 you can have WBFM up to
1300 MHz from HF, why reinvent the wheel when its ready made from China at little more than the
cost of a kit, ???
Also should you take any note of advise given to you by many people to use an LNB , either stand
alone or a Bernie Box many of the VHF/UHF handhelds will cover 600~700MHz WBFM and other modes
The other advantage to using LNBs over a diode mixer with only the antenna gain in front of it is
and extra 30dB of rx gain with a NF below 1dB.

LNBs and cheap Chinese handhelds, or more up market Kenwood and others handhelds, is Cost, time and
easy availability. and still can be done as a club project as the assembly of the front end can be
managed by any of them.

No having to VAT on delivery plus the collection charge by the postal service, Once you purchase
over about £25 you will hit these charges, Between £12 ~ £25 depending on courier/postal service
You can still use a Gunn or simar type Tx, or they can pay around with some cheap LNBs by turning
the fets the FETs, So you have a low cost Tx, you could do a group purchase of pucks.
There are some ancient LNBs around that will pull in to the 10 GHz band , and they can be picked up
for a very low cost.

--
--
Best wishes / 73
Richard Bown

Email : richard@g8jvm.info
HTTP :http://www.g8jvm.info
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