The M0CVO HW-20HP Off-Centre Fed Dipole, RadCom Review, 01/2012, Steve Nichols G0KYA
Multiband antennas can be a bit of a compromise. Long doublets, G5RVs etc can have complex radiation patterns on the higher bands, which can make it a little bit of a lottery as to whether the station you wish to work is on a lobe (good) or in a null (not so good).
Having a second shorter antenna for the higher HF bands, can often make a lot of sense. So I was pleased to be asked to test the HW-20HP Off Centre Fed Dipole (OCFD) from Nigel M0CVO.
OCFDs, sometime inaccurately called Windoms (which technically only have a single wire feeder) are "Marmite" antennas – people seem to either love them or hate them.
I have had reasonable success with a 132ft 80m version, so I was interested to see how the HW-20P performed. The antenna is 33ft (10.14m) long and is split one third/two thirds (6.76m and 3.38m) where it is fed with a impedance transformation balun (more of that later).
This is obviously a half wave dipole for 20m. The idea is that this feedpoint is where the impedance on the antenna's fundamental resonant frequency is between 200-300 Ohms, so giving a low SWR when fed through a 4:1 or 6:1 balun. You can normally also expect the antenna to work on 28MHz (2x 14MHz) as well with a low SWR.
Nigel says that his antenna will also work on 20, 15m, 12m, 11m and 10m without an ATU and on 17m and 6m with an ATU.
He says: "The balun is based loosely around the Ruthroff design, but has been varied somewhat from the available designs into one of my own. I believe that the transformer action of this then also helps somewhat in the operation of the HW-20HP on the 15m band."
The balun is fitted inside a flame retardant ABS plastic box with a tongue and groove fitting to prevent ingress of both dust and water. It has the obligatory S0239 socket on the bottom. Screw lugs are fitted to the sides whereby the two grey insulated wire elements (supplied) connect to the balun inside. The wires are then doubled back on themselves, secured by cable ties and then attached to the box via two key ring-type retaining rings that take the weight off the connections.
This is simple and effective. The far ends of the antenna elements are connected to two plastic dogbone insulators. Heat shrink tubing is used on all joints and the quality of workmanship is good, considering the low price of the antenna.
The antenna is designed to take up to 400W – the "HP" in the title referring to high power.
On my installation I fitted a choke about 10m down the coax to prevent any common mode currents flowing back to the rig. This can be problem with OCFDs and no common mode current problems were noticed with the choke.
The instructions state that the balun is a 4:1 design, but measurements with an analyser and a 200 Ohm resistor showed that the transformation is not quite 4:1. Nigel says that he is planning to patent, or at least register, the new design to prevent copies so it wouldn't be fair for me to comment further about its construction, other than to say that it definitely looked like a Ruthroff design wound on a T200-2 toroid.
The antenna was installed as an inverted V for the test with the apex at 10m and the ends sloping down so that one ended up about eight metres above ground and the other at about five metres. It was fed with about 25m of RG213 coax.
My 100ft doublet was taken down during the SWR measurements and tests to avoid interaction, although the results didn't change very much when it was put back up.
As you can see from the SWR results the antenna could be used many bands without a tuner, although I would recommend using one. In practice any internal ATU will match this antenna down to an SWR of 1:1 very easily on any of the bands from 20m-6m. You will no doubt find that your actual SWR results are different, possibly higher if a shorter length of coax is used.
So how did it perform? Actually, pretty much as expected. Modelling with MMANA-GAL showed that the antenna radiates broadside to the wire on 20m-15m, and breaks into a multilobe clover leaf pattern on 10m and higher.
When compared with conventional half wave dipoles at the same height the antennas were pretty much neck and neck, as you would expect. A long list of stations worked would not really tell you anything.
Sometimes the HW-20HP would have the edge, sometimes my dipoles would, as they are orientated in different directions. At no time did I find a signal that was way down on the OCFD.
It also seemed to work equally well on all of the bands under test. What it did do was outperform my longer doublet at times, which confirmed my fears that these are not always optimal for the higher bands. Nothing was heard on 50MHz during the test period.
I did notice that the antenna was a little noisier than dedicated half-wave dipoles, possibly due to pick up on the vertical coax section.
So overall, the antenna is a cost-effective way of getting on the five or six higher HF bands. It is inevitably a bit of a compromise, but not as much as you
might think. My thanks to Nigel at M0CVO antennas for allowing us to test the HW-20HP. Nigel can be contacted via http://www.m0cvoantennas.com/ or telephone 07921639978.
(At end of feeder and without an ATU)
Please follow this link to the review of the HW-40HP (pdf file will open in new page)
Nigel M0CVO has built up a reputation over the past few years for making and selling cost-effective HF antennas. I featured his HW-20HP off-centre fed dipole in an earlier issue of RadCom and was quite impressed
In fact, the antenna was left up here at my QTH for an extended test and worked well on all bands from 20m to 10m, although you do need an ATU for 17m.
I think my log says it all really with contacts into ZL, VK, and VE7 on CW during the recent Commonwealth Contest, plus DX confirmations over the past year from Amsterdam Island (FT5ZM), China (BG2AUE), Burundi (9U4U) and Burkina Faso (XT2TT) among others. I have been very happy with its performance.
And the antenna isn't really even in an ideal position – it is mounted as an inverted V about 8.5m high at the top of a 10m fibreglass fishing pole, hidden in a tree to keep it as stealthy as possible.
So when I was offered the chance to try M0CVO's HW-42HP off centre fed dipole I was eager to try it.
This new antenna is designed to operate on all bands from 40m (7 MHz) to 6m (50 MHz). M0CVO claims it will operate on 40, 30, 20, 17, 15, 12 and 10m without an ATU (SWR <3:1) plus 6m with an ATU.
He says it is also possible to operate on both 80m (3.5 MHz) and 60m (5 MHz) via a good ATU, although the performance will be down.
The antenna is designed to handle 400W key down (CW) or 500W PEP (SSB).
The starting point of the antenna is the classic off centre fed dipole. The overall length of the antenna is 20.28m or 66ft, with a feed point 1/3 of the way along. This gives two legs of 13.52m and 6.76m and a feed point impedance of around 200 Ohms.
A 4:1 balun of M0CVO's design is added at the feed point to bring the impedance to something closer to the 50 Ohms required to match to coaxial cable, such as RG8 or RG213.
This classic OCFD design would ordinarily allow low SWR operation on 40m, 20m and 10m, but then Nigel has added a twist.
On the long side of the antenna he has added a separate element that is 3.38m long and connects to the feed point. This element is supported about 30cm under the main wire by two pieces of white 15mm PVC tubing, of the type found in plumbing stores.
The antenna may be mounted horizontally, as a sloper or as an inverted V. And if space is at a premium M0CVO says it may also be “bent” to fit in, with no loss in performance.
The balun is fitted inside a flame retardant ABS plastic box with a tongue and groove fitting to prevent ingress of both dust and water. It has the obligatory S0239 socket on the bottom with stainless steel screw lugs fitted to the sides whereby the grey insulated wire elements (supplied) connect to the balun inside.
The wires are doubled back on themselves, secured by cable ties and then attached to the box via keyring-type retaining rings that take the weight off the connections.
This is simple and effective. In the HW-20 version I have been using for the past couple of years I have had to repair one of the connections at the feed point, but this was due to the antenna whipping around in the gales that seem to hit the UK with ever-increasing regularity. The repair was a simple solder job and took less than 15 minutes.
The far ends of the HW-42HP antenna elements are connected to two plastic dog bone insulators. Heat shrink tubing is used on all joints and the quality of workmanship is good, considering the low price of the antenna.
I tested the balun by placing a 200 ohm resistor across the terminals and measuring the SWR with an MFJ-269 analyser. I measured 1.4:1/1.5:1 across the whole 3-30 MHz range, which suggests that the impedance transformation is not quite 4:1. This is not uncommon and I have seen other so-called 4:1 baluns that behave in a similar fashion.
I mounted the antenna on the pole used for the HW20, with its Apex at about 8.5 m with the ends coming down as an inverted V – the longest element ending at about 2m high.
Nigel suggests that the end of the second shorter lower element (where it is attached to the white tubing support) can be tied with nylon fishing line to the end of the antenna to keep it taut. I would agree with that idea, otherwise it can tend to fold back on itself.
Once erected (and fed with about 30m of RG213 with a 10-turn choke balun at the base of the pole) it was time to take some SWR readings. The table shows the results, but as you can see it was below 3:1 at all frequencies of interest and therefore within the range of internal ATUs.
The actual SWR at the feed point is no doubt higher, but a length of coax (and its inherent losses) will result in an apparent lower SWR figure at the rig end. Your figures will no doubt be different with your installation.
The overall design has been designed to work on as many bands as possible and this it does remarkably well.
So how does it perform? Actually, very well indeed. On back to back tests with dedicated dipoles it was usually equal to or no worse than one S -point down on just about every band. On HF it performed almost identically to the HW-20 OCFD it replaced.
In this installation it was quite a quiet antenna (especially on 21 MHz and higher), no doubt helped by getting it as far away from the house as possible and feeding it with a choke balun.
It certainly isn't a compromise antenna – I've seen plenty of antennas that offer a 1:1 match, but are deaf on receive and poor radiators. This one is “lively” and each band was very accessible. A long list of countries worked isn't going to tell you much, but it offers multi-band dipole-like performance in a single antenna and could be a boon for hams without too much space.
But what of that design? A closer examination shows that the 13.52/6.76m legs gives an OFCD for 40, 20m and 10m. The addition of another leg 3.38m long appears to give a second OCFD with a total length of 10.14m and fed at the one third-two thirds point. In other words, an OCFD for 20m.
But life is not that simple – what you actually appear to get is a more complex arrangement that Nigel has obviously optimised to give the best (low) SWR results across as many bands as possible.
What the design did show (and this was backed up by my antenna model in MMANA-GAL) is that although the website suggests that the antenna could be used on 3.5 MHz (80m) and 5 MHz (60m) the SWR is very high on those bands, probably outside of the range of most internal ATUs. The performance is likely to be disappointing anyway as it is too short.
Overall then, the HW-42HP offers effective multi-band performance in a single package at a reasonable price. The antenna costs £56.95 from http://m0cvoantennas.webs.com/ and our thanks to Nigel (Tel: 07921639978) for supplying the review model.
1:1 at 6.298MHz
7.000 MHz – 2.4:1
7.200 MHz – 2.5:1
10.100 MHz – 2.2:1
10.150 MHz – 2.3:1
14.000 MHz – 1.8:1
14.350 MHz – 1.2:1
18.068 MHz – 2.3:1
18.158 MHz – 2.1:1
21.000 MHz – 2.0:1
21.450 MHz – 1.6:1
24.890 MHz – 1.7:1
24.990 MHz – 1.9:1
28.000 MHz – 2.2:1
29.000 MHz – 1.6:1
29.700 MHz – 1.4:1
50.000 MHz – 2.2:1
51.000 MHz – 1.7:1
52.000 MHz – 2.6:1
70.000 MHz – 3.6:1
70.500 MHz – 3.5:1
Two FF plots for the DELTA-15HP, in 3D and 2D. They speak for themselves really.
The Gain of 5.84dBi roughly equates to 3.2dBd, almost equivalent to an HB9CV or 2 element yagi with the advantage that this is in two directions, front and back.
For a full list of VSWR readings, both with and without an ATU please click http://m0cvoantennas.webs.com/Magitenna%20VSWR.xlsx . This will open as an EXCEL worksheet.
Please follow this link to the review of the M0CVO Oscillator Kit (pdf file will open in new page)