M0CVO Antennas Limited

The M0CVO HW-20HP Off-Centre Fed Dipole, RadCom Review, 01/2012, Steve Nichols G0KYA

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.

SWR Results

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)