Making a Shortwave listening antenna

[Guineafowl]

When you are finished, gift it to a mathematical HAM radio operator, one who is interested in antenna design.

I’m hoping that’ll be me one day! But I need to get some builds under my belt, since there’s a big step between reading the ARRL chapter on small loop antennas, and actually building one. Till then, I thought I’d stick fairly closely to your suggestions.

Future projects will include:
- A decent DTV antenna, or at least asking if I can build a better one than is commercially available. It might be that I can’t improve on a bought one, but maybe mass manufacture introduces compromises.

-FM dipole for the workshop radio.

- A loop antenna to put in the window, feedline, then coupling loop to the side of my LW/MW valve radio, which has no provision for an external antenna (and is a series set, so shouldn’t have one). This could be quite tightly tuned to 198kHz LW. By simply placing one hand against the casing, and the other gripping the phone line, I can get an improvement.

 

[Baluncore]

A decent DTV antenna, or at least asking if I can build a better one than is commercially available. It might be that I can’t improve on a bought one, but maybe mass manufacture introduces compromises.

That is a good project, if there is a future for DTV.
It is not hard to beat a very-wide-band commercial antenna, if you know which sub-band you are designing for. Design, build and test for a specific transmit site. Give the design to anyone local, who can build and install it.
I designed a 4 element Yagi for DTV, named after the TX site. The antennas are built in a garage, and installed by a new local antenna business. It gets the best results in difficult locations. The antenna is so simple and low cost, that the profit is made when the client/customer/friend pays the installer to solve the problem. You don't need to advertise a good product and service, you need to employ more knowledgeable people to handle the demand, and then diversify.

 

[Guineafowl]

I’ve had a chance to build the loop as per post #25 by @Baluncore using twinax cable. Even in the daytime, it was receiving some pretty strong overseas signals. Will try later on tonight. One small problem is, the stations don’t seem to announce what/where they are.

 

[Baluncore]

One small problem is, the stations don’t seem to announce what/where they are.

Direction finding can be more useful than station ID.
To separate two signals with the same carrier frequency, rotate the antenna to find the direction of the deep null for the station you want to reject.

There are transmitter listings.
Amateur radio operators use call signs, which can be found listed.
What frequency band are you listening to?

 

[Guineafowl]

6-18 MHz. I’ve found huge lists of SW transmissions, all at different time slots, and it’s quite difficult to work out what’s what with the time differences. I’ll make a rotary stand for the antenna, now I know it works.

Looking at your diagram in post #25, would you simply connect a twisted pair between that and the radio antenna jacks?

I’ve hooked up the VNA, and it shows a resonance around 12 MHz if I tune the cap just right, but it’s not very good (SWR 4 at best) and essentially infinite SWR everywhere else. Given the antenna works pretty well, I assume the tuning isn’t that critical, and there’s some interaction between the radio’s tuning cap and the feedline that isn’t being taken into account.

 

[Baluncore]

Looking at your diagram in post #25, would you simply connect a twisted pair between that and the radio antenna jacks?

You could use a twisted pair, but if the radio antenna socket was balanced, I would duplicate the coupling capacitor and take the differential signal from the two windings.

but it’s not very good (SWR 4 at best)

You could try reducing the value of the 22 pF coupling capacitor.
To get good Q and tuning of the antenna, you must not overload the tuned antenna circuit with the feedline to the receiver.