RA0SMS Miniwhip bad performance on SDRPlay

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df1ech
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RA0SMS Miniwhip bad performance on SDRPlay

Postby df1ech » Tue Oct 11, 2016 2:24 pm

I recently bought a miniwhip in Russia to use on my RSP1 (http://ra0sms.blogspot.be/p/the-active- ... hz-30.html)
This whip is a modified version of the PA0RDT miniwhip antenna and is supposed to be a relative good performer.

A friend uses this same antenna on his RTL-SDR with upconverter and it performs really really well on his rig.

however on my RSP1 it performs not at all no matter what I try only weak signalse lots and lots of QRM. The max performance i get out of it is with the LNA off and gainreduction to it's maximum setting any other setting only raises noisfloor and lowers signals into the noise.

My discone antenna + LNA + gainreduction set to about 50% receives some stations with over 19dB more signal strenght than the whip !
also sometimes a ham band seems empty on the whip while it is extremely busy on the discone.

I'm experimenting for several weeks now with this setup and just can't get it to work good.
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g1hbe
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby g1hbe » Tue Oct 11, 2016 5:31 pm

Have you tried some basic fault-finding, such as checking whether the supply voltage is reaching the unit?

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DanubeBCL
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby DanubeBCL » Wed Oct 12, 2016 7:53 am

Have you tried the antenna on a different receiver to rule out that is is defective?
73, Heinrich

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df1ech
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby df1ech » Wed Oct 12, 2016 4:22 pm

g1hbe wrote:Have you tried some basic fault-finding, such as checking whether the supply voltage is reaching the unit?


Not exactly... I measured voltage out of the feed unit and when I soldered the cable i verified that it was properly conducting and had no shortcut. theoretically that should bring the voltage to my antenna.
measuring at the antenna is difficult but not impossible as I wrapped the connector and bottom of the antenna with https://en.wikipedia.org/wiki/Self-amalgamating_tape

DanubeBCL wrote:Have you tried the antenna on a different receiver to rule out that is is defective?
73, Heinrich


Unfortunately I only have 2 realtek usb-sticks and no upconverter but i'll see if i can arrange something for that as this makes sense to do.
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DanubeBCL
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby DanubeBCL » Wed Oct 12, 2016 5:48 pm

You could open the feed unit and measure the current where the fuse is placed with the Amp meter to get a first impression. If it is not within (estimated) 70 and 90 mA something is wrong in the antenna amplifier.
73, Heinrich

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df1ech
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby df1ech » Thu Oct 13, 2016 11:24 am

DanubeBCL wrote:You could open the feed unit and measure the current where the fuse is placed with the Amp meter to get a first impression. If it is not within (estimated) 70 and 90 mA something is wrong in the antenna amplifier.
73, Heinrich



The fuse is unfortunately a small component which I thought was a Diode but nevertheless I managed to measure a ~80ma power draw by the antenna.
So that seems to be fine.

As soon as my gout goes away in my foot I'll take the RSP to a friend who also has this antenna on his RTL sdr and compare if I get same results as on mine.
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DanubeBCL
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby DanubeBCL » Thu Oct 13, 2016 12:01 pm

df1ech wrote:As soon as my gout goes away in my foot I'll take the RSP to a friend
This is certainly the best solution to find out. 80 mA seem to be OK.
If it should work at your friend's location, I have some thoughts (theoretically):
The Russian electronics uses a dual gate MOSFET. According to my experience the MOSFETs do not have good IMD performance. If there is a strong local FM station (or something) near your location the antenna amplifier itself may be overloaded. Additionally the Russian version uses 20 dB amplification in the MOSFET stage which deteriorates IMD performance even more.
But first you must find out how it performs at your friend's location.
73, Heinrich

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F1BJB
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby F1BJB » Mon Oct 17, 2016 10:02 am

df1ech wrote: The max performance i get out of it is with the LNA off and gainreduction to it's maximum setting any other setting only raises noisfloor and lowers signals into the noise.

Hi
From that description I would suspect some kind of UHF oscillation of the miniwhip.
Try replacing the Q2 emitter follower transistor with something like a 2N2222

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Scannerz
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby Scannerz » Tue Jan 17, 2017 10:25 pm

Bought a RA0SMS mini-whip and managed to get it working with my SDRplay v1.

At 12v DC its poor and a mass of noise, Though at 4.5v DC it came alive with good to pretty good reception and low NF from around 5mhz to 27mhz.
Gain setting was 40 no-LNA for SDRplay with HDSDR. Definitely needed ground lead to mini-whip.
And another ground lead to the SDRplay (SDRplay mounted in a brass box). Both grounds helped a lot.

MWave/Broadcast band needed a gain setting of 78 no LNA (Lowest) in HDSDR,Then the whole band became very receivable.
At 4.5 volts it draws 20 to 30mA. So my 12v battery pwr source should last for ages.

I started by adjusting the voltage from 1.5v upwards , Then the noise floor jumps up then drops to a lower noise floor as
the mini-whip kicks in at around 4.5v. Anything higher than 5v and it gets noisy again.
More experiments with the system this week.....cheers Tony.

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DanubeBCL
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Re: RA0SMS Miniwhip bad performance on SDRPlay

Postby DanubeBCL » Wed Jan 18, 2017 9:06 am

Scannerz wrote:At 12v DC its poor and a mass of noise, Though at 4.5v DC it came alive with good to pretty good reception and low NF from around 5mhz to 27mhz.
But with 4.5 V the intermodulation performance of such amplifiers is not good. Normally the base and gate biases should even be stabilised and adjustable to obtain maximum IMD performance. With 4.5 V both semiconductors certainly run outside their specs. Commercial whip antennas use 30, even 50 V power supply. My homebrew whips run with 20 V.
In the meantime I wonder why there are so many "noise" problems with the RA0SMS antenna. Should there be a design problem? Self oscillation could indeed be one of the problems. Self oscillation in the GHz range indeed increases the noise of the amplifier.

Besides all this: The setup of an active E field antenna is not trivial. It is susceptible to electric noise, needs a good grounding at the base of the mast and the coaxial cable can drag noise from the house to the E field probe.
My experiences with E field antennas over the last five years are:
1. Place them as far away as possible from noise sources (houses).
2. Put them on top of a metal(!) mast appr. 8 m above ground. Less height deteriorates VLF...MW reception, more height deteriorates SW reception due to worse grounding (cable resonances). The ground plane of the amplifier on top of the mast is connected to the metal mast.
3. Both the mast and the cable shield at the base of the mast must be connected to a good ground.
4. In the house (next to the RX) insert a common mode choke into the coaxial cable.

But this all of course does not help when the amplifier itself is unstable or when the receiver overloads due to simplified design of the input stages.

73, Heinrich

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