Re: SDRuno bias-t and Loop Amplifier +

leslie norton

I found there was not much difference between the RSPDx bias-t and the one supplied.
Im using half of the twin sky type cable which is 75ohm and have measured the voltage at the Dx and the loop end (70ft away) and there is no loss in voltage.

One thing that does concern me though isn’t that its tx protection.

I have an MFJ1708BSDR that protects the SDRplay ok but i do worry about the protection if i use the supplied bias-t

Especially when i sometimes knock out 200w, I also sometimes use it with my FTDX3000 again which has great protection as it has an Rx port which goes to ground on Tx BUT not sure how protected the BiasT is.

Just another spanner in the works to worry about!!


On 14 Sep 2020, at 16:18, Ray <ray@...> wrote:

[Edited Message Follows]

The loop amp + fed by the bias-t of USB connected sdr receivers seems a good idea, but does have a few pitfalls. My initial experience of feeding the LA+ from my RSPduo was disappointing. Performance was way below what I expected. My setup is a loop made of 10 mm copper water pipe fed with approximately 15 metres of RG58u coax. Here lies the possible problem, and the cause of the poor performance.

I measured the voltage presented to the LA+ at the connection to the amp as 4.49 volts. The open-circuit voltage was 4.69 volts.
I then fed the LA+ from a separate (linear) 5  volt supply, via a bias-t of Chinese origin and the performance improved remarkably.
The measured voltages at the LA+ were 4.78 volts connected, and 5.14 volts open-circuit. The higher voltage delivered by the separate 5 volt PSU and bias-t obviously made all the difference.

The situation could be improved by the use of RG8 coax, but there still might not be a high enough voltage to power the LA+ adequately. I do not have the facility for varying the voltage to the LA+ to determine the minimum voltage required. A much shorter run of RG58 may be OK, but I have not tried this as I need to locate my loop some distance from the operating point. Using a separate bias-t does have the advantage of being able to locate it much nearer the loop. There does seem sufficient signal presented to the RSP when fed by the 5 volt PSU/bias-t arrangement. Note that I have only checked this on LF and MF frequencies, but I would expect similar results at HF.

Any other experiences with this setup?

Ray, G3NKL

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