Follow Up on a previous question- Transmit antenna on HF band

Question

I had asked a question in a previous segment and the link is provided below Antenna Size limits at 2-30 MHz (10 m to 150 m wavelength)

I think i have some more clarity on the question I wanted to ask. I would like to know if such an antenna is possible with the following criteria

1. SWR of less than 2.5 for entire HF band(3-30 MHz), without any antenna tuners
2. Radiation efficiency that is good enough for transmission( of around 10-20 W)

I have seen T2FD antennas which have radiation efficiencies of around 50% and are wideband in the entire range, but their size is very large(> 30 m) length. Is it possible to design a transmit antenna of around 10 m(or less) at its maximum, and still have reasonable efficiency and SWR of less than 2.5 for the entire HF band.

Answer 1

An antenna under 10M in size with 30% efficiency and a bandwidth (without any frequency adaptive tuning required) from 1 to 30 MHz is likely impossible due to the Chu Harrington limit on Q.

Roughly, using a Chu limit rule-of-thumb, the bandwidth of an antenna goes down with its volume, or the cube of its maximum dimension. So your example antenna would only have a bandwidth of 29Mhz/(3^3) or around 1 MHz, if reduced in size by a factor of 3 (from 30M down to 10M). Going from 50% down to 30% in efficiency by allowing additional resistive losses would most likely not increase the SWR bandwidth back up by the factor of 29. The added resistive losses required to meet your untuned 2.5:1 SWR bandwidth of 29 MHz would likely turn your (or any similar) antenna geometry into a near approximation to a dummy load, with maybe on the order of 1% efficiency... or less.

A low SWR from a small antenna (relative to wavelength) will require precise tuning with a narrow bandwidth for any useful efficiency. Like a mag loop. Or be extremely lossy and thus inefficient. Like a dummy load.

Answer 2

A loaded vertical would work, but you may need to change taps on the load when you change bands. The vertical can be almost any height; but the taller the better. The loading coil can make up for the lack of height for tuning, but the efficiency gets worse with shorter antennas. There's a couple of commercial loaded verticals with calibrated loading coils.

Similarly, a small loop can easily do all the bands, but you have to tune it with the cap built into the antenna. There was an article about a two turn small loop in QST a number of years ago as well.

And then there's always the zig zag random wire antenna, but that probably requires a tuner, especially if you want all band.

But if you want no need for a tuner, you need to look at fat wire antennas, cages, and disk cone like antennas, and most of those are not small.

Answer 3

Well, it depends upon how you define 'good enough'. A log-periodic antenna can be power-efficient and cover the entire HF band with a low SWR, but such an antenna is large and expensive.

A tunable antenna, such as an antenna that uses a motor to reel and unreel a metal tape inside a tube such as a SteppIR, or a "screwdriver" mobile antenna that uses a motor to adjust a loading coil, can cover the entire HF band with a low SWR, as long as you don't mind a short wait to tune the antenna after you change frequencies. But if the antenna is to be power-efficient and cover the 80m band, it must also be large. Unfortunately, there is no easy way around the efficiency problem. "Screwdriver" mobile antennas aren't very large, because they are sized to fit on moving vehicles, but their efficiencies on 80m are terrible. I haven't experimented with a screwdriver antenna myself, but I wouldn't be surprised to hear that typical efficiency on 80m was 1-2%.

A small "magnetic" loop transmitting antenna can be compact. The problem there is that there is a tuning capacitor that must be adjusted when the frequency changes, and also transmitting a reasonable amount of power at the lower frequencies requires enormous current in the loop. This means that the more resistance the loop has, the less power-efficient the antenna will be. I don't know what typical efficiencies are, but I suspect they are also quite low. Careful construction with premium materials helps a lot, from what I've heard.

As I said, there is no easy way around the efficiency problem. But the magic of ham radio can help make up for this. QRP aficionados enjoy the challenge of transmitting with low power; the usual definition for QRP is 5 W or less. You might be surprised to know what QRPers can pull off. 100 W or 200 W into an inefficient antenna can produce the same amount of radiated RF power as 5 W into an efficient antenna. I can tell you from personal experience that sometimes propagation can really surprise. My first European QSO (from the West Coast of North America, which is usually a difficult path) was gray-line propagation using 11 W CW to an omnidirectional vertical on 20m, with a ham in an apartment building in Estonia, at the bottom of the sunspot cycle. Not quite QRP, but close. He was clearly intelligible for a few minutes, I made the contact, and then he vanished. Wow, what a thrill that was! Propagation like that isn't common, but it's not rare either; it's there if you look for it.