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SDRuno bias-t and Loop Amplifier +


Simon
 

My ccw preamp (as have the others .) have protection diodes fitted.

I fitted them to my ccw . There was provision on the ccw pcb to fit..

Makes no difference to imd’s etc.

Simon


leslie norton
 

Not concerned about the SDR as its protected but as the preamp is a “bolt on” the preamp but that gives me confidence Thanks Simon

On 15 Sep 2020, at 20:09, Simon <@Zen> wrote:

Well

I run 400w.. 15m away? Sdr fdm duo trx..

Are you concerned about the preamp or sdr?? No issues with preamps..my sdr is protected internally. One would hope yours is? Maybe check??

Its easy to add a couple of diodes across the input to sdr ASLONG as the sdr is not putting out 5v..?

Simon


Simon
 

Well

I run 400w.. 15m away? Sdr fdm duo trx..

Are you concerned about the preamp or sdr?? No issues with preamps..my sdr is protected internally. One would hope yours is? Maybe check??

Its easy to add a couple of diodes across the input to sdr ASLONG as the sdr is not putting out 5v..?

Simon


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!!

Les
G4JNW

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


Ray G3NKL
 

Hi Chris,
Yes, that can be a problem, but we don't have any control over the voltage on the USB port. It is what it is! Unfortunately I don't have a USB hub here to try although I might try other solutions. You may have noticed I've ordered one of your bias-t units, and also a low noise power supply, although I don't actually
need that. My old trusty home built 12/5 volt PSU with 7812 and 7805 regulators is just a bit large for where I want to put it.

With the higher supply voltage, the LA+ is working remarkably well and compares favourably with my Wellbrook loop. It's propped up against the fence at the moment and there's a chain-link fence immediately behind that.

Ray, G3NKL


Chris Moulding
 

Hi Ray,

As you have found the minimum voltage for the Loop Antenna Amplifier + is just below 5 V.

The USB voltage output from some computers may be lower than 5 V and in that case an external USB hub with a separate 5 V power supply is a good solution.

There is an improvement in gain and IP3 performance if the amplifier is fed via a bias-tee with voltages up to 12 V.

Regards,

Chris


Ray G3NKL
 
Edited

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