Matching an external antenna to the Belka-DX?

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N3NEI
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Matching an external antenna to the Belka-DX?

Post by N3NEI »

I've been enjoying a Belka-DX receiver for more than a year now, and it's remarkable how well it works with its included whip antenna. This seems to be because the receiver is optimized for the whip, as explained here: http://www.merseyradar.co.uk/belka-dx/ With other portable receivers, I've always seen an improvement by adding some extra wire to the attached whip; with the Belka, it's surprisingly hard to outperform the whip.

I recently salvaged about 10 meters of wire and have been playing around with a few configurations inside my third floor apartment -- a random wire, a loop, and a helical vertical wound on a broomstick. Configuring the wire as a loop laid out on the floor isn't as effective as the whip; the random wire and whip are about equal. The vertical is more interesting: tightly wound, 10 meters of wire takes up less than a third of the broomstick and doesn't perform very well. But wrapped in a wider spiral that takes up the whole broomstick, it's at least on par with the whip and presents a lower noise floor. (Any ideas why this would be?)

Unlike the typical 50 Ohms most radios like, the Belka-DX has an impedance "higher than 10 kOhm," according to Alex, the designer: https://swling.com/blog/2020/12/docksid ... -receiver/ All the antenna concepts I've encountered over the years assume one's aiming for a 50 Ohm load, so I'm curious what adjustments should be made to accommodate a 10 kOhm impedance? I'm guessing most matching transformers/baluns/ununs don't make sense, since they're trying to bring high impedance down to around 50 Ohms. Would an external antenna work best with a simple tuner to optimize the antenna's resonance?
Egil - LA2PJ
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Re: Matching an external antenna to the Belka-DX?

Post by Egil - LA2PJ »

I'm guessing most matching transformers/baluns/ununs don't make sense, since they're trying to bring high impedance down to around 50 Ohms.
All the devices you mention are in fact RF-transformers, and they work both ways. Just by swapping the input/output connectors they all work as step-up devices.

I have a Belka-DX myself, and it works happily with a 12m long wire antenna as long as a counterpoise wire is also connected to the ground side of the antenna socket. I use a 7m length.
In the pioner days of transistor SW portables, another trick was used: a small capasitor of around 10 - 30 pF was used, with one end to the antenna input and the other to the wire antenna. I once tried this by just twisting two short lengths of insulated wire, and it worked fine. Don't be afraid to experiment!
N3NEI
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Re: Matching an external antenna to the Belka-DX?

Post by N3NEI »

Egil - LA2PJ wrote: Thu Jun 09, 2022 11:35 amAll the devices you mention are in fact RF-transformers, and they work both ways. Just by swapping the input/output connectors they all work as step-up devices.
Oh, of course -- good point!
Egil - LA2PJ wrote: Thu Jun 09, 2022 11:35 am I have a Belka-DX myself, and it works happily with a 12m long wire antenna as long as a counterpoise wire is also connected to the ground side of the antenna socket. I use a 7m length.
In the pioner days of transistor SW portables, another trick was used: a small capasitor of around 10 - 30 pF was used, with one end to the antenna input and the other to the wire antenna. I once tried this by just twisting two short lengths of insulated wire, and it worked fine. Don't be afraid to experiment!
Interesting about the capacitor. I'll play around some more with a counterpoise, too.
Good post, thanks.
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Andrew (grayhat)
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Re: Matching an external antenna to the Belka-DX?

Post by Andrew (grayhat) »

As a note, the "belka" isn't so picky about impedance (and a lot of oldtime rigs won't present 50 Ohms as their input impedance), no, the issue (if we want to call it so) is mainly about input levels; start by setting the antenna gain to min if you aren't using the stock whip, and, when it comes to a longwire, loop or whatsnot, ensure to match it to your coax; an antenna presenting (just to say) an impedance of 450-j100 at its feedpoint may be directly connected to coax, sure, and it won't do any bad to the receiver (given it's RX only), but then... why wasting signal along the coax, when we're seeking to receive it ? Also, and again, a high input impedance isn't a bad thing, for a receiver.

So what, you may ask... well, start considering the use of decent UnUns or BalUns to couple your antennas to the coax feeder, and, if the antenna is an unbalanced one (like e.g. an endfed - aka a wire laid out someway), give it a "return path to (RF) ground", that is, use a so-called "counterpoise"; be it a single wire dropped on ground or a complex system of radials; in both cases, you'll be closing the circuit (a generator connected with a single pole won't work, it will need a return path for current), improving the antenna efficiency and, in turn, your reception

my 2 cents

P.S.

Forgive me for typos but I'm posting from my phone and I lack sleep (last night slept 2 hours) and I am not a "youngster" anymore :D

[edit]

fixed typos :D
13dka
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Re: Matching an external antenna to the Belka-DX?

Post by 13dka »

^^ What he said. :)

I've used the Belka with very good results with various random wires, active and passive large loops and active whips... The only exception with the Belka is when the antenna is very lossy (has very little output) like an NCPL (e.g. the YouLoop), then the mismatch leads to rather poor performance.

Another reason for less than stellar results is local noise of course. How to tell if there's (wideband) noise? Luckily the Belka has a signal meter that indicates the input signal directly (so it's not affected by RF gain settings) and as a general rule, any signal indication >5 (a few dots on the meter) on an empty channel on let's say 19m AM or 20m SSB on the stock whip would indicate elevated local noise levels.

If there is noise (and there likely will be some), the most wonderful thing about the Belka is that you can wear this little box full of performance in a pocket and walk over to some less noisy place, a park, a meadow or just away from the houses to let it do its thing - sounding like you're dragging a random wire behind you. :)
N3NEI
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Re: Matching an external antenna to the Belka-DX?

Post by N3NEI »

Like the typical home, mine has its share of RFI. That's one reason the loosely-wound helical vertical was interesting, because the noise floor was lower. (But to make that antenna permanent, I need a dedicated broomstick, not the actual household broom!)

I have taken the Belka outdoors on several occasions, including to a remote countryside hilltop, and conditions there were so quiet it was almost as if the radio had a squelch engaged. The random wire (strung along a wooden fence) was terrific there, although the whip wasn't far behind. Apparently, going portable was the motivation behind the receiver's design, so it makes sense that it really shines in that setting.
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Andrew (grayhat)
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Re: Matching an external antenna to the Belka-DX?

Post by Andrew (grayhat) »

NEI3ZKI wrote: Fri Jun 10, 2022 9:44 am Like the typical home, mine has its share of RFI. That's one reason the loosely-wound helical vertical was interesting, because the noise floor was lower. (But to make that antenna permanent, I need a dedicated broomstick, not the actual household broom!)

I have taken the Belka outdoors on several occasions, including to a remote countryside hilltop, and conditions there were so quiet it was almost as if the radio had a squelch engaged. The random wire (strung along a wooden fence) was terrific there, although the whip wasn't far behind. Apparently, going portable was the motivation behind the receiver's design, so it makes sense that it really shines in that setting.
If you can, try some (or all) of these antennas

http://www.kk5jy.net/LoG/

http://www.kk5jy.net/rx-loop/

http://www.kk5jy.net/porch-loop/

if you don't want to wind the 9:1 BalUn used for all the above, you can just pick a NooElec v2 BalUn

https://www.nooelec.com/store/balun-one ... bones.html

which will fit perfectly (just add an enclosure); I believe you'll be surprised at what those passive antennas will be able to pull in and at the reduction of noise level they'll offer (in particular the first one) 8-)
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Andrew (grayhat)
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Re: Matching an external antenna to the Belka-DX?

Post by Andrew (grayhat) »

Oh and, as for "matching", if you feel you need such a thing, you may just pick an MFJ-956, it won't break the bank and will serve you honestly well, plus, if you'll then decide you'll need a preamp, you may pick up whatever wideband LNA and place it between the MFJ-956 and the receiver so that you'll have a "tuned preamp", that is, the preamp will only "see" (and amplify) the signals filtered by the preselector :D
Dave613
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Re: Matching an external antenna to the Belka-DX?

Post by Dave613 »

I have used a Belka DX outdoors and recently compared the supplied telescopic with a 75 foot circumference loop on ground. My tests have not been rigorous or extensive but it was immediately apparent that there was a clear advantage to the LOG in SNR. It was enough that on a weak signal it could make the difference between copy/not copy.
I used 28ga silicone insulated stranded wire and Nooelec 1:9 balun along with jumper and adapters as required to match connectors.
All that and Shure 215 IEMs and the Belka will fit in the small case of the type described here: https://swling.com/blog/2022/03/a-case- ... nary-tale/
Dave.
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