Bias T and active mini-whip

Useful information regarding antennas for SDR products.
Paul
Posts: 76
Joined: Fri Feb 17, 2017 9:52 am

Bias T and active mini-whip

Postby Paul » Fri Jun 30, 2017 10:49 am

Having watched Kevin Loughlin's video (https://www.youtube.com/watch?v=F0MD51FqPPY&t=979s) building an active mini-hip from kit and hearing the audio quality produced when powered by his RSP2 Bias T, I was encouraged to try this and was pleased with the result, obtaining a very clear and clean signal, comparable with that on the video.
Needless to say I was quite "chuffed," being able to dispense with the external psu, it's associated leads/cables and achieving a neater installation as a bonus.
Having gone back to it today however, selecting the antenna followed by powering up the Bias T has no effect, any signals received being by the apparently unpowered RG58 cable run and whatever else.
I would really like to use this option if it would work reliably without causing problems.
Any explanation or advice would be much appreciated.

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g1hbe
Posts: 214
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Bias T and active mini-whip

Postby g1hbe » Fri Jun 30, 2017 10:56 pm

I've noticed something odd about the bias T. If I have had it ON and then shut down the receiver, when I start up again it says the bias T is still powered but it is not. To make it work I have to turn the T off then on again.

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Andy

Paul
Posts: 76
Joined: Fri Feb 17, 2017 9:52 am

Re: Bias T and active mini-whip

Postby Paul » Sat Jul 01, 2017 6:59 am

I hadn't noticed that Andy - but it did come back and disappear again - don't know why at the moment, but I'll give it a whirl and see if it does the same here.
Thanks for your time and reply.
Cheers
Paul

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DanubeBCL
Posts: 168
Joined: Sat Jul 02, 2016 12:39 pm

Re: Bias T and active mini-whip

Postby DanubeBCL » Sat Jul 01, 2017 8:37 am

A question in this context (I do not own an RSP2):
Is the bias T voltage of the RSP2 "clean"? I have a Funcube pro+ and it's bias T is a horror. It can only (if ever) be used for VHF and above. Below is carries extreme USB noise and noise from the 5 V USB line. I have applied extra SMD caps on the 5 V line, but no avail. As soon as the bias is on, high broadband noise covers the whole SW band (even when no active antenna is connected). You better leave it switched off on the Funcube.

But, on the other hand: I would recommend to feed an active mini whip amplifier with at least 12 V, better 15 V, not with 4.7 V for reasons of IMD performance.

73, Heinrich

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g1hbe
Posts: 214
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Bias T and active mini-whip

Postby g1hbe » Sat Jul 01, 2017 6:53 pm

Heinrich - I've just checked the noise on the Antenna B input with the bias-T enabled/disabled. I can detect nothing, even with the scope on 5mV with a x1 probe. Clean as a whistle. Of course, this may just mean my particular laptop is clean!

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Andy

Paul
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Joined: Fri Feb 17, 2017 9:52 am

Re: Bias T and active mini-whip

Postby Paul » Sat Jul 01, 2017 8:08 pm

Thanks for your time yet again Heinrich.
The try with the Bias T was inspired as I said by the really impressive performance of Kevin Loughlin's commercial assembly.
His RSP2's Bias T sounded incredibly clear and crisp - I can't really speak for my own (RSP2) as there does seem to be a problem (the mini-whip that I'm using is much the maligned RA0SMS, that actually performs OK here on the 12v Stable 1200ms brick that I have - although it IS switched - I do have another much more serious job - a 13.8v/5-7Amp that I was thinking of using - it's a serious bit of kit from a previous endeavour,
It does seem like overkill, but it's not currently being used for anything else.
As the RF isolating transformer requires 12-14V/100ma I think that it ought to do as it will only draw what it requires I believe?
Thanks again
Paul

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DanubeBCL
Posts: 168
Joined: Sat Jul 02, 2016 12:39 pm

Re: Bias T and active mini-whip

Postby DanubeBCL » Sun Jul 02, 2017 7:40 am

g1hbe wrote:Heinrich - I've just checked the noise on the Antenna B input with the bias-T enabled/disabled. I can detect nothing, even with the scope on 5mV with a x1 probe. Clean as a whistle. Of course, this may just mean my particular laptop is clean!
OK, that's better than the Funcube. I just fetched my Funcube again (not used for a long time) and tried the bias. Nothing but noise on SW up to 30 MHz when the bias is on. I always wondered why they provided the bias. But as it's clean on VHF/UHF I assume it is intended for preamps there, e. g. a PGA 103+ (AMSAT etc.).
@Paul
I recommend to use 12...13.8 V for the whip and the bias T which came with the whip. 4.7 V may work, but it is too low to feed the amplifier correct. As soon as your whip is outside a building on top of a small mast the whip will produce high signal levels and has little reserves when fed with 4.7 V.
73, Heinrich

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Paul
Posts: 76
Joined: Fri Feb 17, 2017 9:52 am

Re: Bias T and active mini-whip

Postby Paul » Sun Jul 02, 2017 9:42 am

Thanks for your no doubt good advice (again) Heinrich - at the moment it's back on the 12V/1200ma (switched) brick, which I suspect may be problematic.
It was great for 2 hours or so yesterday evening and again this morning, equaling or better than the 20M longwire, then the power level very obviously dropped off dramatically and removal or reinstatement of the power supply made no difference.
It would seem suggestive of overheating perhaps and at present, i would bet on the supplied RF isolating transformer, but I suppose that it could also be the whip itself, which is now cable-tied to the top of an 8M fibreglass pole.
I will be changing to the 13.8V / 5-7 Amp as soon as I get the necessary connectors sorted out.
Best Wishes
Paul

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DanubeBCL
Posts: 168
Joined: Sat Jul 02, 2016 12:39 pm

Re: Bias T and active mini-whip

Postby DanubeBCL » Sun Jul 02, 2017 10:00 am

Paul, with a power consumption of 100 ... 150 mA nothing should overheat. Either you have a (sporadic or permanent) short circuit or an interruption on the power supply line somewhere or there is indeed something wrong in the whip.
Do you have an Amp/Ohm/Voltmeter to make measurements on the equipment? Without it is difficult to find the culprit.
From the schematic of the ra0sms I see there should be a fuse in the bias T. Maybe this fuse is blown.

BTW: With the bias T unit of the ra0sms connected you should never activate the bias T in the RSP. The RF isolating transformer in the external bias T unit will short circuit the supply voltage of the RSP. A damage of the RSP could happen!
73, Heinrich

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Paul
Posts: 76
Joined: Fri Feb 17, 2017 9:52 am

Re: Bias T and active mini-whip

Postby Paul » Sun Jul 02, 2017 5:58 pm

It's pretty surely a sporadic power fault - having opened the RF isolating transformer, from it's appearance I suspect that it has very likely overheated and I would guess that what I have been experiencing have been random short circuits.
At least the RSP is still delivering the goods and I must say I find it's functionality pretty impressive.
I don't have access to test equipment and in any case insufficient knowledge and skills to sort it out, so I'm putting it out of use until I resolve the issue in one way or another.
All I would say, is that when the whip worked it WAS impressive and I will try again - at least I have rediscovered a decent power source.
Back to the longwire and discone for a bit it is for the moment.
Thank you for the advice Heinrich, you have been most helpful.
Best Wishes
Paul

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