Re: SG-Labs,


Ian White
 

A more modern spreadsheet version of Tcalc is downloadable here:
http://www.ifwtech.co.uk/g3sek/vhfdx/dg8-design-calcs.xls

The pre-loaded example is for 2m, but the same principles apply on microwaves too.

For further details, 'The VHF/UHF DX Book' is now a free download from
http://www.trpub.net/html/dx_book.htm


73 from Ian GM3SEK


On 05/11/2020 12:11, SAM JEWELL via groups.io wrote:
I have previously recommended running either GM3SEK's Tcalc or the old but still very useful Appcad programs to analyse what your system parameters should (could?) be. One word of caution. With AppCad, anyway, cascaded stage intercept numbers do not always stack up due to some cancellations. So I always leave a few dB uncertainty in the calculated dynamic range figure. Noise figures are generally pretty close.

The VLNA preamps are high gain (on70 and 23cm) and moderate on 13 and 9cm. Their output intercept is quite reasonable and may, in many cases, exceed the input intercept of many transverters and rigs such as the IC9700.
Use masthead gain with care. In most cases you do not need to have better than about 1dB noise figure in terrestrial systems, so you may be able to trade off some low noise performance agains loss in a prefilter. Do watch that some preamps have very poor input match and will therefore not terminate filters very well. This can lead to instability in the preamp and even excess loss in the misterminated filter.

73 de Sam, G4DDK





------ Original Message ------ From: "Robin Szemeti - G1YFG" <robin@...> To: UKMicrowaves@groups.io Sent: Thursday, 5 Nov, 20 At 11:39 Subject: Re: [UKMicrowaves] SG-Labs,
I had to put attenuators between my DDK preamp and the SSB electronics transvertor, as even with the cable loss and the preselector filter, there was still far too much gain. The excess gain was OK when the band was quiet, but a real problem during a contest.
On Thu, 5 Nov 2020 at 09:34, Martin Phillips G4CIO <martin@...> wrote:
Yes and no re the preamps.
If you're using the preamp with a low NF transverter or receiver (like the SG labs tvtr) you can get away with quite a lot of cable loss after the preamp. With a mediocre nf receiver you'll still need low loss coax to avoid degrading the nf by much.
10 dB should be enough gain in the SG labs tvrt with its low quoted nf unless the following rx is very poor indeed.
Martin/
On 4/11/20 10:19 PM, militaryoperator via groups.io wrote:
The only trouble with SG Lab tranverters is the low rx gain, 10db. The Kuhne ones have 20db.
The SGL amp for 23 and 13 has an LNA but again, only 10db.
I think Mr H really needs to improve that on his next designs.
There's also a problem with timing on the rf v ptt switching, if you run the pa off the ptt there's a double kick in the pa dropping and going back to rx.
I have the 23, 13 and 9 units and they are very good save for the above caveat.
Ben G4BXD

-- Robin Szemeti - G1YFG

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