Friday, June 27, 2014

Inverted V Aerials

Another option maybe to support the dipole at its centre on a tall pole, or roof apex, with each end sloping downwards to lower fixing points. This will give the aerial an upside down V shape. As with a sloper, the Inverted V arrangement will give the aerial some directivity - a different radiation pattern compared to a straight horizontal dipole.

Spectrum Communications G4CFY Trapped Dipole in Inverted V configuration

Spectrum Communications G4CFY Trapped Dipole in "Inverted V" configuration

Using an Inverted V can help fit a dipole into a slightly restricted space. The Inverted V arrangement can be used for single band resonant dipoles, trapped dipoles and fan dipoles.

Inverted V (ref

At A, details for an inverted V fed with open-wire line for multi-band HF operation. A Transmatch is shown at B, suitable for matching the antenna to the transmitter over a wide frequency range. The included angle between the two legs should be greater than 90° for best performance. [ref:]

W3DZZ antenna by the Maidstone Amateur Radio Society


W3DZZ antenna by the Maidstone Amateur Radio Society that adds a dedicated 10 meter (28MHz) resonant element as a 'fan'.

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

Moonraker supply a whole range of wire trap dipoles covering from 2 to 5 HF bands (MTD1; MTD2; MTD3; MTD4; MTD5; MTD6). Diamond also produce trapped wire antennas, the W-721, W-728 and W735. Comet and Diamond each produce similar interesting 5 band wire dipoles that utilize both traps and a fan arrangement - the Diamond W8010 and the Comet CWA-1000. If space really is limited then look out for KZJ Communications (dongo1950 on ebay) - he produces 'Limited Space Inductive Dipoles'. These are inductively loaded and shortened dipoles so they will have reduced efficiency, of course, but are very nicely made, so might be very useful in a tight spot.

To obtain good efficiency and achieve a low angle of radiation, desirable for longer distance DX, a horizontal dipole needs to be installed at a good height - over 20 feet would be desirable and it is quite common to install horizontal dipoles at around 30 to 40 feet above ground level. This might be a problem at some QTH's, it certainly is at mine!

Allan Copland, GM1SXX comments: "The dipole will operate well on the band it has been sized for , if placed at a suitable height, but will also operate as a’ three-half-wave’ aerial at three times the frequency and so on, so it’s not strictly a single band aerial.  An 80M dipole (132 feet typical) will work nicely on 30 metres  (three half waves) but not on 40m (two half waves)… because on 40M the feed-point  is at a voltage node and not at a current node, for easy feeding.  Most aerials are current fed.

The radiation pattern changes when a dipole is not used on its design frequency. The pattern will break up into multiple ‘petals’. This can be either a disadvantage or an advantage depending on what you expect from it.  Since most of us use co-ax, an UN-BAL  should really be used to connect the unbalanced feeder to the balanced aerial, but how many people actually bother? Not many I suspect.  It’s possible of course to use a balanced feed-line  system instead with a dipole and just have a delta match (no centre insulator… none needed).  There are many choices and permutations, but in general, dipoles are centre fed at a point of current maximum (and minimum voltage).

A normal dipole is current fed but of course can be voltage fed instead. This is what’s done in the EFHWA or Fuchs aerial where a resonant half wave wire is fed at one end (max voltage / min current) from an L/C tank, against a very short counterpoise wire.

Adding 160m / Top Band to an Inverted L


The 160 metre Top Band can be added to this aerial by connecting a 3.5 MHz trap at the end of the 80 metre wire (where to monofilament joins the 6.55m section of wire below) with another length of wire on the other side, increasing the overall length of the antenna.
Find out how to do it here:

Adding Top Band to and Inverted L Antenna

Adding Top Band to an Inverted L by Len Paget GM0ONX (Practical Wireless magazine)

Inverted L - 80 metres to 10 metres


A typical Inverted L antenna will be trapped for 40m/80m using a 7.1 MHz trap. It is essentially one half of a W3DZZ dipole so can be accommodated very much more easily into a small plot or garden - especially as part of the antenna is running vertically up a wooden or fibreglass (non conductive) pole. This should allow it to be fitted into quite a small garden such as mine.

The Inverted L is also a very effective aerial because it has the benefit of both vertical and horizontal radiation. While Inverted L's might make good TX aerials, like ground mounted vertical aerials they can be quite noisy on RX.

The Inverted L is extremely easy to 'home brew'. Spectrum Communications can also supply the complete aerial as shown below. It should give excellent performance on 80m and 40 metres, with 20 metres also being good but allowing use on 15m and 10m and possibly one or two of the WARC bands:

Spectrum Communications 40m / 80m Trapped Inverted L

Small Loop for 20 metres to 10 metres:

A loop for 20 metres or 17 meters is relatively compact and could easily be installed in small 'postage stamp' sized gardens. A loop antenna could be triangular, square (Quad) or circular, but a square loop (and indeed a circular loop) would need more supporting points than a delta (triangular) loop, so a Delta loop is likely to be the easier option.

The loop is really a single band antenna cut for one wavelength on the band of interest, however it can also work quite well as a cheap and easy to install multi-band H.F. aerial. A loop consisting of a 17 metre length of thin antenna wire, for example, will work well on 17 metres but may also give 15m, 12m and 10m with an ATU. My own loop is made from an 16 metre length of wire, tuned for the 17m band, but can work on higher bands. A 40 metre loop will be considerably larger, but it might still possible to accommodate in many fairly compact gardens. Performance will depend on height and orientation.

Feeding the loop at the top or bottom will give horizontal polarisation, while placing the feed point on the side will give vertical polarisation. The apex can be at the top or the bottom, but performance should be better with the apex at the bottom with the flat wire across the top  - however for ease it may be more convenient to support a Delta Loop on a single pole, meaning that the apex would be at the top.

Ideally a loop should be fed with balanced line back to the shack, connected to a balanced line ATU or other ATU via a 4:1 balun. Alternatively use a 4:1 balun at the antenna end and run 50 ohm coax back to the ATU / txvr - though losses will be greater doing it by this method if the coaxial cable is quite long.

If one can install a separate antenna for the lower frequency bands of say 160m, 80m and 40m, then a Loop Antenna could be a good partner to allow operation on the higher bands of 20 metres to 10 meters or even 6 metres.

A loop should be really very easy to install using a single support pole and very cheap too! All that's needed is the supporting pole, some cheap wire, a 4:1 balun which can be 'home brewed' and some thin cord and insulators which should not be an eyesore either.

Multi Band Delta Loop using 4:1 balun at feedpoint

Diagram from the excellent article by W5SDC