Short wave DX'ing

[Hybrid tellies]

For short wave DX'ing I have abandoned my mains operated radio with a wire aerial in the loft and instead use portable radios with their built in telescopic aerials and running on their internal batteries.

Its not ideal but I can hear stations such as the VoA from their transmitters in Botswana or Sao Tome, the BBC World Service from many of their overseas transmitters such as Ascension, Madagascar, Oman and Singapore, CRI with very clear signals from their Kashi transmitter site, Radio Romania International, REE from Spain and the Voice of Greece. More recently I have heard Radio New Zealand on the 41m and 25m bands.
I also like to receive the BBC World Service from the UK site at Woofferton. As I am well within the skip zone the signals are rather weak with strange flutter fade effects.

I do enjoy DX'ing across the broadcast bands as I study radio wave propagation a hobby I have been involved with since my school years.

I think the secret in reducing the noise levels is isolate your radio from the noisy mains power supply as much as possible by using internal batteries. I would like to invest in a Wellbrook magnetic loop aerial or possibly have a go at building a Wellgood aerial.

[Jimbo]

As other contributors have pointed out, there is a significant reduction in Shortwave Broadcast transmissions by comparison with ten or twenty years ago. Running a radio transmitter at 200 or 500 kilowatts for 24 hours a day is an expensive business even for some of the heavyweight players. Many of the 'Broadcasters' are still there, but their programme content is streamed on the Internet alongside much more information than can be carried by simple Amplitude Modulation of a carrier wave alone.

There is, however, plenty still to hear, and in some respects, better opportunities to hear some of the less powerful and more distant stations now that the big guns are silent.

It does need a bit more of a strategic approach in order to work out the times of day and times of year to listen, quite apart from the selection of aerial and receiver as noted in earlier posts.

A very good starting point is to look at the Internet pages of the British DX Club, [ www.bdxc.org.uk ], where a sample copy of their monthly journal 'Communication' can be viewed. The reception logs at the end of each month's edition list the catches noted by members of the club, sub-divided into the various sections of the Broadcast Bands spectrum, [all of which exhibit different behaviours and characteristics], together with details of the time of day, the programme content, the strength of reception and the equipment used by the listener.

The BDXC also print a twice-yearly summary of the 'Broadcasts in English' that can be heard during every hour of the day.

I hope this is of some interest.

Jimbo M0GJD

[dave walsh]

My son got me a subscription to the Idler magazine for Xmas. I only realised this when the Jan/Feb issue turned up! To my surprise Page 74 Slow Tech Station To Station is an article by Lee Osborne recommending SW Listening as a hobby. He calls it the "dark web" of SW listening, unusual perspectives and spies sending code messages [Numbers Stations I suppose?]. As he is listening to Radio North Korea I can see what he means perhaps He points out that it's not an expensive hobby and that you can get a set for £40 [much less in fact]. Or that you can listen for free using online radio services, although I'm still trying to work out whether that's a rather contradictory approach He says that quite a few SW Stations still remain but notes that listening to totalitarian states isn't everyone's cup pf tea. He mentions Radio Romania International, as an example of a country disseminating Arts and Culture on SW. The article is well written and entertaining overall. At the ends he recommends an American You Tube offering, Shortwave Radio-The Dark Web Of The Airwaves In 2020, which describes and demonstrates what's out there! It's "One for the brave " he says about... instrutables.com/Listen-to-Shortwave Broadcasts-on-an -AM-Radio which gives advice on conversion.

Dave W

No connection clearly but I was surprised by the quality and content of this [post Private Eye] mag overall. Eg there is an article explaining why non alcoholic beer has never quite tasted the same but a new process overcomes that. It's all in the yeast apparently

[Radio Wrangler]

Originally Posted by Bazz4CQJ

I think I've seen at least one design where someone used varicap diodes to tune a loop.

Best avoided.

Varactor diodes are fast, very fast. Any strong signals will modulate the capacitance of the diodes at their frequency. So the tuning of the loop shimmys and modulates everything. So intermod products appear and spoil your reception of smaller signals. They make the band appear full, but many of the signals aren't actually there.

A loop tuned with a mechanical capacitor and a motor is inconvenient. On the other hand it works as a great preselector and protects your receiver from being tenderised by big signals., AND it reduces the total bandwidth of noise hitting it.

Varactors are so fast that they are used as UHF/microwave frequency multipliers and as the active ingredient in microwave parametric amplifiers. As a two-terminal device, they can only see one voltage, and can't segregate tuning control from signals and vice versa.

David

mechanical capacitor

A switched (reed relays?) capacitor bank maybe (just like antenna auto tuners but lower power) with an added magnetic repulsion trimmer cap., repulsion is easier to control than attraction.

[ManxDave]

Originally Posted by Radio Wrangler

Best avoided.

Varactor diodes are fast, very fast. Any strong signals will modulate the capacitance of the diodes at their frequency. So the tuning of the loop shimmys and modulates everything. So intermod products appear and spoil your reception of smaller signals. They make the band appear full, but many of the signals aren't actually there.

A loop tuned with a mechanical capacitor and a motor is inconvenient. On the other hand it works as a great preselector and protects your receiver from being tenderised by big signals., AND it reduces the total bandwidth of noise hitting it.

Varactors are so fast that they are used as UHF/microwave frequency multipliers and as the active ingredient in microwave parametric amplifiers. As a two-terminal device, they can only see one voltage, and can't segregate tuning control from signals and vice versa.

David

David thanks for pointing out the problem of the signal affecting the capacitance of a varicap/varactor. After reading Bazz's post I ordered a K1561A-3 which seemed to have a wide capacitance range ( if I can work out whether connecting the individual diodes in series or parallel gives the widest range ) . Now I have it I may as well give it a try despite the problem you have pointed out. Will let you know how I get on once I have built a suitable variable PSU. It looks like it will be back to a stepper motor driven mechanical variable cap though.

[Bazz4CQJ]

Originally Posted by Radio Wrangler

Best avoided.

Varactor diodes are fast, very fast. Any strong signals will modulate the capacitance of the diodes at their frequency. So the tuning of the loop shimmys and modulates everything. So intermod products appear and spoil your reception of smaller signals. They make the band appear full, but many of the signals aren't actually there.

A loop tuned with a mechanical capacitor ...

David

While the rationale that a strong signal in the loop could modulate the varicap sounds sensible, I wonder just how strong such a signal would need to be have noticeable effects? Taking a quick look at a handful of internet sites discussing varicap tuned loops, the comments are generally favourable and problems with modulation are not reported, but I'm willing to believe that such problems might not be identified easily. I suppose that rotating the loop to null a particular strong station might be revealing. I'm struggling to recall whether any of the commercial loops (for amateur receiving use) are tuneable by whatever means?

B

[Bazz4CQJ]

On the issue of active loops with varicap tuning, I stumbled upon this fairly recent designed (2018) published in the ARRL mag "QEX". Not sure whether this would work with a loop. However, I wonder if it might be good to use with a vintage domestic radio with a shortwave band that may "need some help" ?

B

[murphyv310]

Hi.
I agree with David regarding varactor diodes. Tried this with my tunable frame aerial on medium wave and had images all over the band. In fact even with a tuning cap and 0A91 feeding a set of headphones if you spoke into the phones you'd hear your voice on an adjacent radio placed near the frame over the station you were listening too!

[Radio Wrangler]

It's unfortunate, but with reception problems involving intermodulation and harmonic distortion, the appearance is created of a greater number of stations on the band than there actually are. There is a temptation to interpret this as "Look how many more stations it's pulling in!"

Things on HF have got easier, though, since the ranks of high power broadcasters started closing down.

Using a genuine, high dynamic range receiving system doesn't have a wow factor, it just seems quieter. You start to find there are gaps between stations, then you start to find weak signals in those gaps.

So it can be quite difficult for someone to spot problems in this area, you just don't know that you're missing something because you can't hear amidst a lot of background-level signals which aren't actually there. It takes some doing to convince people of this as well.

David

[Bazz4CQJ]

Yes; as per my post#27, I still have not found a commercial design for ham use that uses a tuned loop. My Wellgood seems secure (apart from the time I cut its coax with my hedge trimmers).

B