Minitune Front End

Discussion about the MiniTiouner board and the MiniTiouner Digital ATV Receive Software. See also http://www.vivadatv.org/index.php
g0mjw
Posts: 2330
Joined: Sat Sep 20, 2014 9:15 am

Re: Minitune Front End

Post by g0mjw » Thu Apr 05, 2018 1:53 pm

Yes, you will need filtering. Its very important and no more than the usable bandwidth would be a good guide, i.e. 3MHz on 2m and 8 MHz on 70cm. Ideally, just the band segments 146-147MHz and 434-438MHz but practically you may want to use the antenna for phone or CW.

The gain of commercial preamps is typically around 10-20dB which ought to be more than enough for the SERIT with the built in preamp. You may need more for other tuners. Minimise the gain to avoid hitting the dynamic range and make sure the filtering is as early in the receiver chain as possible. On 2m you could even have the filtering in front of a preamp in the shack* if the filter and cable loss is low enough. More likely though, something like a DG8 from GM3SEK/G4DDK which has good filtering included. On 70cm one of the SSB preamps, e.g. SP-70, would be a good choice. They also have filtering built in, but typically 20MHz to allow for the wider band in region 2.

*You don't have to use a masthead preamp on 2m. If your cable loss is under a dB you might was well put the preamp in the shack as there is little point getting the system noise figure below 2dB unless you have a very quiet location. Typical urban locations these days are so noisy that it is questionable if a preamp is required below 70cm.

Commercial preamps are expensive. They are not hard to build. Mini-kits http://www.minikits.com.au/ do a range of kits. You can design and build your own, but if so, I suspect you would not be asking this question.

Mike

Richmmm
Posts: 2
Joined: Thu Apr 05, 2018 8:35 am

Re: Minitune Front End

Post by Richmmm » Thu Apr 19, 2018 9:15 am

Thanks for your reply...........

Do you or anyone know of a switched dual band preamp?

I was thinking of something similar to the DG8 preamp on 2m, with something suitable for 70cm, switched by relays (or solid state switches) between the two bands. I would keep the DG8 designed relay switches to provide a straight thru path on Tx (although I am not sure how well they would work at 70cm). I was planning on using 13V/18V on the coax to power the amps, and switch between bands, which should make interfacing with the Minitioune Rx easy. Also a simple Bias Tee could be used if not powered from the Minitioune.

One more question: I note the DG8 2m preamp is not sealed in an RF tight metal case. In my situation, located close to a TV broadcast mast, I would guess I might need more screening?

Rgds Rich

G4FRE
Posts: 197
Joined: Fri May 26, 2017 6:57 am

Re: Minitune Front End

Post by G4FRE » Fri Apr 20, 2018 8:18 pm

I have built a couple of the dg8 dual input relay systems for an HF project. They have .48db loss and 52db isolation at 432Mhz as opposed to 0.05db loss and 74dB isolation at 144MHz
Dave
G4FRE

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