RA0SMS vs PA0RDT Active Mini Whip - RA0SMS is Very Noisy - Why?

[DanubeBCL]

So if you put it on metal pole the pole acts as an common-mode-choke and the "antenna" stops working.

That's not true. Active monopoles work fine both on top of metal masts and non-conductive masts.

because the coax cable is part of the antenna.

This is true. That's why an active monopole is always a compromise. The higher it is mounted above ground the stronger the signals are (especially on VLF...MW), but on the other hand the coax cable as the counterpart of the plate/whip shows more and more resonances on the shortwave bands which make some frequencies appear stronger, others weaker.
All owners of mini whips have been going through long periods of experiments to find out the best compromise for height above ground and size of the probe element. Those only interested in VLF...MW put it on masts appr. 8 m above ground (or more) and ignore the bumpy frequency responce on shortwaves. For a balanced frequency response for all bands (VLF...SW) a height of appr. 3 m above ground seems to be ideal. In both cases the probe consists of 1 m rod or appr. 80 cm² metal plate. Smaller probe sizes will of course give different results.

Also, the bigger you make the plate/wire the worse electric spikes are fed into the receiver.

This is partly true. When the probe is bigger and you live in a noisy environment (true for most of us), both signal and noise rise. The SNR remains the same. A big plate/rod increases the danger of receiver and/or amplifier overload.

The plate/wire isn't the real antenna, the coax is mostly.

The antenna is the sum of plate/rod, coax cable, amplifier/impedance transformer and grounding. An active monopole is a delicate antenna. I needs some experiments to find the optimum dimensions for the individual location.

73, Heinrich

[ON5HB]

Have fun: https://owenduffy.net/antenna/PA0RDT-MiniWhip/

[ultravista]

Any recommendations for fixing this problem?

[ON5HB]

From what I see you have 3 problems.

1 the RA0SMS is essential the same as the PA0RDT and because the "antenna" section is longer it will pickup more noise and it looks like it's overloading the SDRplay.
So you probably need to reduce the RF-level a lot.

2 The power-supply being very poorly build. This can be simply tested, use a 9V battery and power the antenna's instead of the power-supply.
If the heavy noise is gone then it was the power-supply also causing rubbish.
Feeding badly stabilized power into an active antenna/amplifier is a major problem for receiving signals.
As a battery is perfectly stabilized, it's the best way to check.

3 Make common-mode-choke outside your house, experiment where it works best, to keep in-house-noise from the coax.
Can be made with a plastic-bottle and a piece of coax, search for ugly-balun, you will find plenty ideas.

[Mike2459]

This can be simply tested, use a 9V battery

Wouldn't this change the Q point of the amplifier?

[Paul]

As a previous user of this antenna, albeit, the previous copper-plate version, after it's psu failed, I used the Bias-T 4.5V supply successfully.
Couldn't detect any difference in performance (which I'm not going to comment on).
It may be worth your looking though...

[ON5HB]

This can be simply tested, use a 9V battery

Wouldn't this change the Q point of the amplifier?

It will amplify a bit less, shouldn't be a problem.
Many others do feed these antenna's with a battery.
Also my mini-whip said in the manual that any power between 5~12V should be fine.

The point is to find out if the noise is also generated by the powersupply or not, if it's gone on a battery, then you know the PSU is not good for this purpose.

[Mike2459]

After looking at the screenshots (which are very familiar - I've had the same problems with the PA0RDT MW) It appears Andy has put forth the cause of the distortion. Ultravista said the antenna is 20' AGL. I'm not optimistic, but would lowering the antenna to say 5' or so (as a test) be worthwhile?

[Mike2459]

I have two local MW broadcasters, one on 1170 (-51dbm) the other at 1370 (-36dbm), their signals mix and appear at the sum (2540K Hz) and difference frequencies (200Khz). At the A+B and A-B freqs the audio of both stations overlap. In an attempt to minimize this, I first trimmed the E-plate on the whip, (10X30mm was removed). It lowered the output level across all bands, of course, but the but A+B and A-B was still there. Next I lowered the source resistor in the 1st stage amp of the miniwhip from 680Ω to 330Ω - this eliminated A+B and A-B most of the time, it occasionally appears but much weaker. The downside to this is that the current draw of the miniwhip is increased. I am now within about 20db of the receiver noise floor at LF on minimum noise days (around -130dbm). At 28Mhz the floor is closer to -140dbm. The 10Khz spaced spikes still appear but are reduced in amplitude considerably.

[Mike2459]

ultravista wrote:

The RA0SMS produces noise approximately every 10 KHZ - the noise is present from LF all the way through HF.

Since reading this I have encountered similar noise spikes at various times and on different portions of the LF/HF spectrum. I have taken measurements of the frequency of these spikes. In every instance the spikes occurred at almost exactly 8000 Hz intervals (plus or minus 1 to 2 Hz). Using the CW mode of the SDRPlay it's possible to get relatively accurate measurements. ultravista wrote "approximately every 10 KHZ". Using HDSDR it may be difficult to distinguish the difference by observation alone.

Andy wrote in reply to above

Peaks every 10 kHz sounds familiar - where are you located? If you are in the USA, then this may be intermodulation from MW broadcast stations which are spaced by 10 kHz.

.

I live in the US and in my opinion this may not be the direct cause.