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There seem to be reports of people trying to see how far an HF DX contact can be made using the tiniest transmit power (mW or even sub-mW QRP). But many of these contacts seem to be accomplished using fairly full-sized antenna installations (beams, towers, etc.)

What's the equivalent of QRP DX for minimum antenna size rather than transmit power? Assuming a typical, rather than QRP, transmit power, say 100W, what's the smallest antenna over which a HF DX, say at least 1000 miles distant, contact can be made? Do mobile operators manage to do HF DX with relatively short car roof or bumper mounted whip antennas? Or with something even more portable? Has anyones "dummy load" been clearly heard at a DX distance?

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In the August 2010 QST article, "Who Are You Calling a Dummy?", Page 46, Steve Ford says:

And speaking of veteran hams, these same old timers will also be quick to remind you that dummy loads do radiate. It's all a matter of degree, after all. If you are pumping 500W into a dummy load, chances are at least some of the RF energy will escape. Amateur Radio lore is replete with tales of contacts made with dummy loads - and even between dummy loads!

As such, your question might be answerable by asking you how small a dummy load you can get, and how good the receiver is at the other end of the DX, and when you can find the best propagation conditions.

The problem with asking for "the smallest" is that if anyone gives you an actual dimension for the supposed smallest HF antenna, you will have to ask them to prove a negative - that you can't DX on anything smaller.

You can't prove that, though.

At the moment there's little interest in the community in building a "small antenna" equivalent to QRP. Part of the problem is that antenna size depends directly on frequency, and gets larger the lower frequency you go. It would be difficult to come up with a size that applies to all bands as an upper limit to count as a "small antenna" Perhaps one could come up with a table, or simple calculation. Further, does weight factor into it? How do you count environmental aspects - does using a small antenna on a large metal roof where the radio is grounded still count, even though the roof makes a better ground plane than someone able to complete the contact on the ground? With QRP, there's only one measurement, it's valid across all bands, and performs a useful function in encouraging people to make better use of the spectrum and invent new techniques to run lower power.

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  • $\begingroup$ Here's one account, written humorously, of making a contact (not DX, though) using a dummy load: QST Sep 2002 Pg. 20 "Who is the Dummy?" $\endgroup$ – Adam Davis Feb 12 '14 at 21:15
  • $\begingroup$ Link seems to require an ARRL login. $\endgroup$ – hotpaw2 Feb 12 '14 at 21:20
  • $\begingroup$ If you want to see contacts made with dummy loads, you don't need tales: just go to any hamfest. The radio manufacturers will put dummy loads on their demo equipment so thousands of attendees at hundreds of booths can all play with the equipment without jamming the entire band. $\endgroup$ – Phil Frost - W8II Feb 12 '14 at 21:35
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Just as there is no lowest power for QRP, there is no smallest antenna. As antennas get smaller, they tend to approach, but never reach 0% radiation efficiency. If a smaller antenna has say, 50% radiation efficiency, you can compensate by doubling the transmitter power. To anyone receiving your signal from some distance, there will be no observable difference.

If you can't double the transmitter power, then you can use a modulation technique that doubles the energy per unit of information. For example, you could switch from AM to SSB, or you could halve the bitrate for a digital modulation. If you can't do that, there's something else you can do. See How can I know over what distance or at what speed I can communicate?

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  • $\begingroup$ Theoretically yes. Practically, in terms of a (non-April-1st) QST/QEX article, or a well documented HF DX QSL card, there's probably a lower limit at which none can be found. $\endgroup$ – hotpaw2 Feb 12 '14 at 21:44
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    $\begingroup$ @hotpaw2 that's like asking, "Can two hams with no legs named 'Jason' make contact on a Tuesday?" and then concluding, because no one can find such a QSL card, that it's impossible. Neither "QRP" nor "DX" have rigorous definitions, so any threshold you might define is necessarily arbitrary anyhow. $\endgroup$ – Phil Frost - W8II Feb 12 '14 at 21:49
  • $\begingroup$ I specified 2 thresholds in my question, 100 Watts as typical non-QRP for HF, and over 1000 miles as DX. For size, I'll take the maximum dimension of the antenna, assuming that most of the transmit power is actually radiated from that antenna, and not parasitically from elsewhere. $\endgroup$ – hotpaw2 Feb 12 '14 at 21:59
  • $\begingroup$ Also, asking what is on record as possible is a completely different question from asking for a proof of impossibility otherwise. $\endgroup$ – hotpaw2 Feb 12 '14 at 22:06
  • $\begingroup$ @hotpaw2 between your comment about the QST article, and the question itself, I can't decide if you are talking about theory or record. Anyhow, I can think of dozens of likewise unanswerable questions: What's the farthest a sailplane can travel? What's the most rainfall a location can receive annually? What's the fastest one can travel around the world? All of these things are limited by stochastic processes, and there's a point at which success becomes unlikely, but it's a fuzzy, undefinable threshold subject to luck, weather, current technology, operator skill, and other random variables. $\endgroup$ – Phil Frost - W8II Feb 13 '14 at 3:34
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Tom N6BT reports accomplishg "Worked All Continets" on 10m CW using 120 watts and an incandescent light bulb as an antenna.

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  • $\begingroup$ One thing that might be a potential issue with the N6BT report is all the other high gain antennas in the near vicinity. Could a tiny parasitic coupling from the transmitter wiring to any of the other antenna feedlines have possibly led to more radiation from those big antennas than from the lightbulb? $\endgroup$ – hotpaw2 Apr 1 '14 at 7:52
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A magnetic loop antenna (magloop antenna for short) (example) can be a good choice for high efficiency in a small size if it is designed well.

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  • $\begingroup$ Please see How do I write a good answer? in the site's help section. I strongly suggest that you edit this answer to elaborate on how using a magnetic loop antenna answers the original question, and why it might be a good choice for the OP's needs. Showing an example of the size and height magloop antenna needed for a given antenna gain and take-off angle on a particular HF band might be a good start. $\endgroup$ – a CVn Apr 11 '14 at 7:24
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You could maybe try a random wire antenna that's made to lie flat or be suspended from your house to a pole at the end of your garden.

Here's a table of the best calculated lengths for such:

http://www.hamuniverse.com/randomwireantennalengths.html

I'm sure there might be some designs floating around that give you specifications for how you might make it smaller, but it's worth a try at the least!

I know a HAM that does what I mentioned above, and manages to get the US and Romania on 10W, easy, but barely make it down the road, lol.

Good Luck!

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  • $\begingroup$ One could maybe try any number of antennas. Why would a random wire antenna be better than any other antenna? An anecdote is not a sufficient explanation, as no doubt thousands of other people have made good contacts on other types of antennas. $\endgroup$ – Phil Frost - W8II Feb 13 '14 at 19:14
  • $\begingroup$ It was just an idea. You don't have to bite my head off. As no doubt thousands of people probably have, but they are generally know to be quite good for those who lack space for a fully blown antenna. If you have town/ country restrictions, there's little point in suggesting a 200FT vertical, is there? $\endgroup$ – AnaCronIsm Feb 13 '14 at 19:31
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Perhaps a better way to phrase the question is: What is the simplest antenna with which you can make DX contacts?

I live in southern California. Since last November, I have been using a 65-foot (21.3-meter) end-fed wire and a counterpoise consisting of five long wires.

The antenna and counterpoise go to a balun outside of my shack. The balun feeds the signal into a 6-foot (2-meter) length of coax that goes to my antenna tuner. I use this arrangement to keep RF out of the shack and to keep the antenna from picking up RF noise from the house wiring and the shack.

My transmit power is usually 70 to 100W.

Thus far, I have logged QSOs with the Island of Jersey, Sicily, Poland, France, and Germany on PSK31. Using CW, I have had QSOs with Japan, South Korea, Alaska, Yukon Territory (Canada).

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  • $\begingroup$ What about this arrangement makes it especially small or suited for DX work? $\endgroup$ – Phil Frost - W8II Mar 29 '14 at 13:08

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