We recently installed a commercial, (Procomm) E-field Whip at work
for HF monitoring/listening. Its on the roof of the building, about
25m high bolted to a metal rail connected to the building steel
structure. But initialialy as an oversight, the ground wire of the
whip wasn't connected to the building frame so teh whip was working
agains teh braid of the coax.. The lab is inside a huge brand, new
metal framed building with just about every modern electrical gadget
there is. The crud from about 2000 PCs, air conditioning, building
management systems, security locks and alarms etc imposed so much
onto the outside of the run of coax going to the roof, there was about
200mV of noise injected and returning down the coax inside, amplified
into the Rx. THe ONLY HF signal that could be heard was the BBC
from Rampisham, about 40 miles away nearly line of sight.
Grounding the antenna to the metal framework of the roof platform,
reduced the noise by something like 50dB, and while its still bad,
amateurs could be heard on 7MHz.
Conclusion, don't install an HF receiving facility on a major
industrial-type building. If you need an electrically quiet HF
receiving facility, put it in a field in the middle of nowhere. Now,
if only our managers could be persuaded of that :-)
Andy
www.g4jnt.com
This email has been scanned for damaging side-effects by the health
and safety police
2009/12/16 Dave G3WCB <[email protected]>:
> Hello, Stefan.
>
> This is very interesting. I have a MiniWhip on a pole some distance from the
> house, and it seems that most of the interference (ADSL hash) seems to be
> re-radiated by the coax outer and picked up by the antenna.
>
> A relatively short optical link of one or two metres may still reduce the
> pick-up, as it would break the "copper" connection to the antenna.
>
> Alternatively, it might be possible to transvert the amplified LF signal up
> to a vhf frequency and then set up a remote RX some distance away.
>
> You could switch the power on and off by using a pressure switch from a
> discarded washing machine connected to the shack by a lenght of 6mm plastic
> tubing. Apply slight positive pressure to turn on the antenna circuitry!
>
> 73, Dave G3WCB IO91RM nr Windsor, S.E. England.
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Stefan Schäfer
> Sent: 16 December 2009 16:51
> To: [email protected]
> Subject: LF: Noise cancelling by using optic transmission of RX signals
>
> Dear Lowfers,
>
> I want to start a new subject (for me it's new)...
>
> The last weeks I still had RX problems, either with the tx antenna or with
> the short E-flied RX antenna.
>
> I found out that the noise was/is generated by my notebook which made RXing
> difficult, even in regions without any else local QRM (what could almost
> bring me to shut down the PC and just do nice and easy CW QSOs without
> anything else! (but most traffic is in QRSS...)).
>
> Then I have done mni tests with many many ferrite cores (that I found in an
> older emc lab in our institute), e.g. 20x FT50-77 plus 20x FT50-43 plus 20x
> FT50-61 and e few others. I placed them all in series, partly in the near of
> the preamp and/or in the near of the RX. Sure, there was a significant
> reduction of noise but lastly the noise remains.
>
> Accidentally, during a test with my VFO connected to the same 12V supply of
> my preamp, I observed a strong line in the argo monitor EVEN without an
> antenna (short wire) connected to the VFO and when turned the amplitude to
> zero! That meant for me, the VFO and its supply is not suppressed enough and
> so some RF comes to the preamp by the coax and supply line! (the preamp is
> supplied by a battery). I thought, I can use this (normally unwanted)
> coupling path to test the blocking capability of the ferrite cores and so I
> tried it with all variations and later with an additional BIG ferrite core
> and many turns of RG174. As I meant, surely there is a significant noise
> reduction but the line in argo was still well visible!
>
> What I have done then (since 4 days) is using a coupling link by an optic
> transmission of the 137kHz signal between the preamp and my RX using a
> SFH750V as the TX Diode and a SFH350 as the RX and abt 20m of a fiber optic
> cable (yes, there are surely more suited components but these were available
> in the moment). These are also used as optic links into high end audio
> systems and therefore easy to get...
>
> Now, when placing the preamp on the same place then during the tests before
> and just changing the coax by the fiber optic cable, the line by the VFO was
> completely gone!! Just with an antenna connected to the VFO and high
> amplitude (and thus e real near-field “connection”) it came back. The
> overall noise level decreased to its absolute minimum during all the tests.
>
> Before I used this optic cable the only signal that could be received (out
> of the city with much much qrm) was DCF39 and DLF and so on. Now, DCF39 came
> out with much better SNR and I saw the first LF Stn at all, it was Ossi /
> OE5ODL transmitting his 5s in the evening (tnx! very nice SNR in QRSS3
> mode).
>
> My conclusion is, that the qrm is always brought to the very sensitive
> E-field antenna by the coax, even when doing many usual suppression methods
> cause there always remains a residual coupling impedance between both sides
> of the cable. Furthermore a long coax cable, say 20m away from the shack
> carries some qrm to the near of the rx antenna so the effective distance to
> the shack is always reduced.
>
> One small disadvantage of this method is the need for a battery supply. My
> TX diode needs abt 20mA and is now working since 3 days with a 7AH Lead Acid
> battery. In the future I plan to use a stereo optic cable. Then, one could
> switch the preamp on and off by the seconds line doing it the opposite way.
> So, smaller batteries could be used, which is necessary when mounting the
> preamp to a shaky fiber mast if one does not want to change the batteries
> each few hours ;-)
>
> Signal distortion seems to be no problem, as I can confirm until now.
>
> Now, my K2 has an optical LF input! ;-)
>
> Perhaps this will give some ideas to the local qrm bothered stns who tried
> everything with ferrite cores, isolating transformers and so on without a
> satisfying result.
>
> NOTE: I do not know if anyone has tried this before and has written a
> publication about that. My goal is not to be the hero in inventions, to
> become popular and especially not to compete with anyone (like it seems to
> be usual in career/job to be the one who gets the pay rise) but just to
> share my ideas to those who are interested to try something new (?). I do
> not say that my solution is the best at all and so on! But, if it will help
> only one Lowfer getting new ideas and improving his station, and if this
> improvement results in more activity on the band, then it would have helped
> all of us!
>
>
>
>
>
> 73s es 55 de Stefan / DK7FC
|
