Making a Shortwave listening antenna

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    Antenna Loop
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Discussion Overview

The discussion revolves around the design and construction of a shortwave listening antenna for an old valve/tube radio, specifically focusing on the use of a tuned loop antenna. Participants explore various configurations, calculations, and practical considerations related to antenna design, including inductance, capacitance, and coupling methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • One participant suggests starting with a small, tuned loop antenna under 0.1 wavelength long, centered on 12MHz, using RG6 coax.
  • Another participant calculates the inductance and reactance for a single triangular loop and proposes a tuning capacitance based on these calculations.
  • Concerns are raised about the design of the triangular loop, with suggestions to consider a two-turn loop for better performance and noise rejection.
  • Participants discuss the importance of tuning the antenna and the potential use of a variable capacitor for adjustments.
  • One participant shares the specific model of their radio and mentions plans to modify the antenna design to a two-turn square loop.
  • Calculations for a square loop antenna are presented, including inductance and capacitance values, with a focus on distributed capacitance effects.
  • Several participants inquire about tools to check the antenna's point of resonance, discussing options like HF antenna analyzers and alternative methods using AGC lines.
  • One participant reports on the initial performance of their built two-turn loop antenna, noting that it does not significantly improve signal strength compared to coax alone.
  • Discussion includes considerations for feedline design, with suggestions for using higher-impedance twisted pairs instead of coaxial cables.

Areas of Agreement / Disagreement

Participants express a variety of views on the optimal design and configuration of the antenna, with no consensus reached on the best approach. There are differing opinions on the effectiveness of different loop designs and the implications of coupling methods.

Contextual Notes

Participants mention various assumptions and calculations related to antenna design, including inductance and capacitance values, but these remain unresolved and depend on specific configurations and conditions.

Who May Find This Useful

Individuals interested in amateur radio, antenna design, or those seeking practical applications of electromagnetic theory may find this discussion beneficial.

  • #31
Baluncore said:
There are HF antenna analysers available when you need one. Expect to pay US$50.
Search eBay for NanoVNA-H Vector Network Antenna Analyzer.
That's seriously cheap. The display on the images implies all sorts of things about what it does. I remember using an h.p. analyser that cost a couple of grand. What does the nanoVNA do different for £50?
 
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  • #32
sophiecentaur said:
That's seriously cheap.
The difference is that the Nano VNA is NOT calibrated to a reference standard.
You can afford to buy one and play with it.
When you are finished, gift it to a mathematical HAM radio operator, one who is interested in antenna design.
 
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  • #33
Baluncore said:
The difference is that the Nano VNA is NOT calibrated to a reference standard.
You can afford to buy one and play with it.
When you are finished, gift it to a mathematical HAM radio operator, one who is interested in antenna design.
OK - no calibration but what about the directional coupling, complex voltage measurements and display? £50?!!!!! I bought a cheap VHF SWR meter for about the same amount and that was £30. It hardly had more than a half dozen components. Very useful though!
The years have made such a difference.
 
  • #34
As above, I’m getting an idea of real resistance from the Smith chart display. I can move the marker along the sweep, to get a readout of the impedance at a particular frequency, in ##R+jX## form.

The nano VNA comes with open/short/##50\Omega## terminations which can be attached to the end of the little test cable in calibration mode. I assume this takes care of the extra cable length.

Twinax cable on order.
 
  • #35
Guineafowl said:
The nano VNA comes with open/short/ terminations which can be attached to the end of the little test cable in calibration mode. I assume this takes care of the extra cable length.
I believe that is the case.

Guineafowl said:
Twinax cable on order.
I would have used some old cat x, network or USB cable, and made a loop from that. Or taken the braid off some old coax, and fed a twisted pair through the centre. You need to get creative and scavenge for things to make antennas. It is both cheaper and quicker than waiting for delivery.
 
  • #36
Baluncore said:
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.
 
  • #37
Guineafowl said:
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.
 
  • #38
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.
 
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  • #39
Guineafowl said:
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?
 
  • #40
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.
 
  • #41
Guineafowl said:
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.

Guineafowl said:
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.
 
  • #42
Love my NanoVNA, had it for a year now
makes tuning up antennas a breeze. It's rated to 4GHz

Also have a TinySA Ultra ( Spec Analyser) rated to 6GHz, still helpful at 10GHz to show signals
just not an accurate signal level

wish these were around in years gone bye when I was doing much more antenna work
 
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  • #43
Baluncore said:
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.


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.
Thanks. Both of these antenna projects have been on the back burner, as life is getting in the way a bit.

As far as I can see from the schematic in post #5, the radio antenna socket is unbalanced, having the lower side connected to mains-earth-ground. I assume I should therefore keep the single 22 pF cap?

In the building of this antenna, I’ve completely missed the ‘ground’ connection at the bottom of the diagram in post #25. Does this mean an extra flying lead to mains-earth-ground?
 
  • #44
Guineafowl said:
I’ve completely missed the ‘ground’ connection at the bottom of the diagram in post #25. Does this mean an extra flying lead to mains-earth-ground?
The design in #25 is balanced (in theory, at least) so there should be no spurious current to flow down any other 'earth lead' to upset the sensitivity pattern or effects from local E fields. Equal and opposite currents flow in the inner and inside the screen of the coax with none on the outside. I don't know why that ∇ symbol is on the diagram; I'd expect the antenna to be insulated from the mast / 'earth', ideally.
 
  • #45
Guineafowl said:
I assume I should therefore keep the single 22 pF cap?
The 22 pF isolates the one-sided cable capacitance from the dual gang tuning capacitor. You may benefit by reducing the value, which will reduce the coupling to the coax, and so increase the Q of the antenna tuning.

Guineafowl said:
In the building of this antenna, I’ve completely missed the ‘ground’ connection at the bottom of the diagram in post #25. Does this mean an extra flying lead to mains-earth-ground?
The earth is there to reduce the pickup of local radio noise from the structure of the building, hull of the boat, or the fuselage of the aircraft.

Static discharge from rain, lightning strikes, or St. Elmo's fire, need to go directly to ground, without following the coax to the radio room.

When you connect the antenna to a receiver, your hand will contact the outside of the coax through the connector. If the antenna is not earthed, someone could receive a shock in the radio room.

During testing, the antenna will be sheltered, so the outside of the coaxial cable braid will do the job for you, while the signal propagates on the inside.

sophiecentaur said:
I don't know why that ∇ symbol is on the diagram; I'd expect the antenna to be insulated from the mast / 'earth', ideally.
The ground symbol connects the screen and the centre-tap of the two loops to the structure of the 'mast'. That parallels the practice of grounding the boom of a Yagi antenna, and the centre of the dipole element. That ground is there to protect the coaxial cable insulation and the operator, from Thor at his best, on a Thursday.