One of the most common questions from any amateur new to the HF bands is, what antenna is the best? This is a question that is almost impossible to answer simply, and you can guarantee everyone has a different opinion! By the way, the word ‘antenna’ is interchangeable with ‘aerial’, they both mean the same!
Generally, a large aerial is best for the low end of the HF spectrum, where the wavelength is long, and a much smaller antenna will give you better results on the higher HF frequencies, where the wavelength is shorter.
This is the main reason many amateurs will use more than one antenna, but just as many will use one aerial, either a large wire one giving better results on the low bands, or a small aerial favouring the higher bands. As they are using just one antenna, they accept it will be a compromise and work better on certain bands than others.
If the aerial is resonant on a given frequency, this means it is exactly the right length. This also means the correct impedance is seen by the transmitter, and all of the power generated by the transmitter can be radiated through the antenna. The aerial is said to be a good match.
If you have an aerial that is too long or too short for the given frequency, the aerial will not be resonant, and the transmitter will not be able to transfer full power, the aerial is therefore a poor match.
Fortunately, if the antenna is not too far away from resonance, it can be matched with a matching unit, otherwise known as an Antenna Tuning Unit.
The same is true for received signals, the longer or shorter the antenna is from the resonant length, the poorer the incoming signal will be unless matched. If the aerial is far too long or far too short it may be impossible to match, depending on the matching unit.
If you have a small garden, the most practical solution will be to have just one antenna. Fortunately, one antenna can be made to have multiple band characteristics, and there are a variety of methods employed to give low impedances on multiple bands. The antenna will still be a compromise on certain bands, but not everyone has the space needed for an Antenna Farm!
In my experience, it can be important for a blind operator to use a low impedance antenna. If the aerial has low impedance characteristics on multiple bands, it is likely the operator will be able to match the aerial using the automatic antenna tuning unit built into the radio. This built in antenna tuning unit or ATU will only have a limited range, and only be able to deal with small impedance variations.
The aerial I have used for many years is the trapped dipole. It is a wire antenna and is T shaped in profile. The feeder rises vertically to a T piece at 40 feet high, and the signal is radiated by the wire element, extended each side of the T piece. The overall length of the wire is 106 feet, or 53 feet each side of the T piece. Each leg of the element contains a 40m trap. This means that moving from the T piece, the wire extends for about 33 feet, there is then the trap, and the wire extends approximately a further 20 feet. On the 40m band, the traps isolate the final 20 feet of wire on each side of the dipole, but the whole length of the antenna is used on each of the other bands. Because of the 40m traps, The trapped dipole can be tuned on multiple bands with an internal Antenna Tuning Unit.
My second HF antenna is a Yagi beam. It is a Steppir 3 element Yagi, and the elements mechanically adjust to the correct length for the chosen frequency. The advantage of a beam is its ability to focus the signal in the chosen direction. Whilst most serious DXers (hams wanting to work long distances) will use a beam, they are relatively large, and require a strong mast and a method of turning or rotating the beam. The Steppir is a 3 element beam on 10m to 20m, and a rotary dipole on 30m and 40m.
The subject of this review is the trapped inverted L with very limited grounding. This antenna is more suited to a smaller garden, and gives access to all the HF bands. I literally wince at the prices of some of the small multi-band verticals, but my trapped inverted L was homebrew.
Basically, the trapped inverted L is equivalent to just one leg of my trapped dipole. The length is 53 feet, and the 40m trap is about 33 feet from the feed point, with a further 20 feet of wire to the end.
The feeder, or in this case the coax, runs along the ground to the base of the aerial. Here we mounted a small connections box on a copper ground rod driven into the earth. The coax is split, with the outer screen connected to ground, and the inner conductor bonded to the base of the inverted L wire.
With all antennas, it is important to get it off the ground and as far from surrounding obstructions as possible. In my case we used the bough of a tree to take the wire vertically up by about 20 feet to a pulley, and the remaining 33 feet was pulled horizontally to another support. If you don’t have a tree handy, your pulley could be at the top of a 20 foot pole.
As you can see, you now have an inverted capital letter L. Ideally, the vertical section needs to be at least 20 feet high, but can be as high as you can get it. The higher it is, the less wire will be left to pull horizontally.
So, my inverted L rises vertically 20 feet. Turns at a right angle and runs for about 13 feet to the trap, and then continues horizontally for the remaining 20 feet.
Ideally, the antenna’s grounding would be much more than one ground rod. I have no doubt it can be improved by adding radials, but this review is to see how the basic aerial performs. If you have a small garden, you will not have the space for radials.
In this RX comparison of the trapped inverted L with my other HF aerials, I have made recordings on each of the HF bands. I have marked the start of the inverted L with one beep, the dipole with two beeps, and the Steppir with three.
On 80m you will hear LY1TR in Lithuania, some 1100 miles. Firstly the inverted L followed by the stronger dipole, and then the sequence is repeated. Interestingly, RA4LHD in European Russia calls in at a distance of 1800 miles, and I think he is slightly stronger on the inverted L.
On 40m we hear GB4C in England, this is a local contact. Firstly he is quite faint on the inverted L, primarily because the inverted L is vertically polarized, and will favour a longer distance. There is not much to choose between the trapped dipole and the rotary dipole of the Steppir which is broadside to the station.
Again on 40m we hear Jr1CFP in Japan at a distance of 6000 miles. The inverted L is slightly stronger than the trapped dipole, and the Steppir is the strongest, rotated towards the station.
On 30m we hear SD7W in Sweden on IOTA EU-138 at a distance of 1000 miles. I think the inverted L is slightly stronger than the dipole.
Again on 30m we hear ZP6CW in Paraguay at 6200 miles. Here, the inverted L is noticeably stronger than the trapped dipole.
On 20m we hear EG5INT on a Spanish IOTA Island with a distance of about 800 miles. Here the inverted L is stronger than the dipole and not far off the beam. We hear the inverted L followed by the trapped dipole, then the Steppir, and finally the inverted L again.
On 17m we hear VK2DX in Australia at about 9500 miles. Following The sequence of three, the inverted L is repeated.
On 15m we hear W7VJ in Washington at 4800 miles. The inverted L is receiving almost as clearly as the 3 element at over twice the height. I’m sure the beam would be showing a higher signal strength, but in terms of clarity there is not much to choose between them.
Again on 15m we hear W0IZL in Nebraska some 4200 miles.
On 12m we hear KU1T in West Virginia some 3700 miles. The inverted L is much stronger than the dipole, but there is noticeably much less noise on the beam.
On 10m we hear 9A3TY in Croatia at 1000 miles. We hear the three antennas, and then the inverted L again. The inverted L is a workable strength, but with heavy QSB on the final clip.
Again on 10m we hear KH7Y in Hawaii a distance of 7200. Faint on the inverted L, no copy on the dipole.
In summary, the trapped inverted L is suitable for a small garden, and is relatively low profile above ground, and has a tiny footprint on the ground. My antenna cost less than the 20m of coax used, and the 40m trap can be bought if you can’t make one. My inverted L was positioned down the garden, and pulled back to the house. I mention this because the horizontal part of the aerial is directly above the coax lying on the ground, and I recall reading somewhere this is desirable. Perhaps the coax acts as a counter poise.
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