Friday, 1 April 2016

Mirfield Electronics Quad Band Antenna, 6m/4m/2m/70cm, and a near miss.

EDIT: For the last word (?) on this topic see http://gm4fvm.blogspot.co.uk/2016/07/mirfield-quad-band-vertical-i-cannot.html

Hi.

The fact that you are reading this at all is thanks to good luck, as we shall see by the end (if you get that far).

I have decided that trying to keep two VHF verticals aloft here is just too difficult. Both my 2m x50 and my 4m 5/8th got bent over the winter, and my 5/8th over 5/8th 2m colinear was wrecked the year before. Also, the appearance of so many verticals (the HF one is 8 metres tall too) draws attention to my presence.

It is interesting that Sirio have released a half wave version of the Gainmaster HF vertical (my present one is a 5/8th) which is 2m shorter than what I have now. I accept that the 1/2 wave version will not perform as well as the 5/8th, but I feel I have to be aware of the appearance of my antenna farm. It cannot be allowed to cause too much of an intrusion onto the skyline here.

Returning to VHF, I have been spoiled by having antennas with good gain. The x50 does very well but it suffers from a low installation. The 4m 5/8ths has the prime site. I have been thinking that if a multi-band antenna with 2m and 4m on it became available then I would put one up.

I did consider building one, but I rejected that idea given the rather extreme wind conditions here.

Enter a new quad band vertical sold by Mirfield Electronics. This claims to cover 6m/4m/2m and 70cm. It is 1.2m long, so I guess it is a loaded (short) quarter wave on 6m, a quarter wave on 4m, a half wave on 2m and some sort of co-linear on 70cms. The claimed gain figures (in dBi) of 2.5/2.5/3/5.5 seem to confirm that.

I think, to make clear at the start, a compromise antenna is bound to be less effective than single band antennas, at least in some respects. And not only is it a multi-band antenna, it is shorter than the ones I have now. I would expect performance to be lower than my present set-up, but as a trade off I will save a run of co-ax, save having two VHF verticals, and hopefully I can put it as high as I can by making just one more elaborate mast rather than two bodged up ones.

So taking 2m as an example, the new quadband antenna is the size of an x30 (a half wave antenna) , but I am replacing an x50 (a 3/4 wave antenna). And the claimed gain figures for the x30 are about 2dB worse than the x50, so this new antenna is bound to be a step back for me on 2m. And that will be even more so changing from a 5/8th to a 1/4 wave on 4m.

Anyway, despite the drawbacks I still think this is worth doing for me. So the quad band antenna arrived yesterday and I got it up the same day. It did not go to full height, it is about a metre below where it will eventually end up (I hope). I got a bit distracted taking down the 6m beam and rotator, so I did not quite get it finished ...
In this view you can see the x50 on the left and the new quad band antenna on the right (the main mast is in the left background). I only got the quad bander just above the ridge tiles before cold/ numb fingers/ rain/ hail/ darkness forced me inside. Later I should be able to raise it at least a metre more.

Construction-wise it is a pretty standard fibreglass-encased tube. The inside the tube is resin filled so I could not peer into it to find out the construction. It has three sturdy radials and the standard enclosed PL-259. The fixings worked for a 35mm mast, and they seem to fit up to a 50mm mast.

I thought it was a nice touch that a compression type PL-259 was enclosed in the pack. That was until I discovered that the 259 on the existing co-ax had the wrong thread for the new antenna. So I had to fit the supplied one anyway. Just as well actually, as when I hacksawed off the old plug the braid had corroded back to where I had cut it. I would have had to change it soon anyway. Then I re-discovered the joys of soldering in a freezing wind with added rain.

PL-259 installed, the antenna was assembled and up it went. There were no instructions supplied but then how could you go wrong? Initial impressions are that it is up to the usual standard for construction of these things.

It is not yet in the clear, but the analyser shows that it resonates in the right places for 4m and 2m. SWR is about 1.3:1. My analyser does not go up to 70cms, but the SWR appears fine. Where I did strike a problem is at 50MHz where it is resonating at about 46MHz and the SWR in the band is 2.5:1. That may be because it is too close to the roof at the moment.

Edit - I have returned the antenna to the seller. More careful measurements and update are here http://gm4fvm.blogspot.co.uk/2016/05/mirfield-quad-band-vertical-update.html

I do not expect sparkling performance at 50MHz, as the antenna is (physically) shorter than a quarter wave. 6m is handy to have though. I do not use 70cms much (though that might change). For me it is really a dual-band antenna. I plan to keep my Anytone and Yaesu rigs and use a diplexer (I use a Comet diplexer in my car at the moment). The small loss caused by the diplexer is something I am willing to accept.

I think that this antenna will appeal to people who, like me, want to minimise their mast numbers, or who own the Wouxun 950 quad band FM rig. If that 6m SWR lowers when the antenna reaches full height then I will be happy enough.

The Mirfield quadband antenna is of modest length and comes with comparable modest performance. It is producing signal reports for me which are down on my previous (larger) single band antennas. I expected that. I still think it is worth the trade off. A 1.2m antenna catches the wind a lot less than the 1.8 and 2.5m ones it replaced, and that is worth having for me.

Now if  an enhanced version appeared which was comparable to the x50 or so, maybe a half wave on 4m and 3/4 on 2m or some combination of 2m colinear (or something), and which was, say, 1.8 or 2m long, then that would be even better for me. OK, longer, but still only one mast and co-ax run to provide. But in a strange way, I might like the shorter, simpler, one for a while.

I did get a bit of a scare taking down the 6m beam before the vertical went up in its place. There was an "largely uncontrolled descent situation". This followed a sort-of pendulum action with the antennas, mast and rotator on one side, and me and the scaffolding tower on the other. The pivot of rotation was around a (very bent) wall bracket and its u-bolt. This u-bolt is now silent key. The sight of the steel bracket bent through 90 degrees was quite something. The mast and rotator horizontal to the ground but several metres up looked a bit odd from my perch at the same level. I had time to watch it, because I was clinging on to the bottom of the mast (which was now the right hand end of the mast). By much heaving I could get the whole thing vertical again, but then I was stuck as the u-bolt trapped the whole thing, even when I took the nuts off it. So we rotated back to horizontal again, several times, as I tried to get something freed up. Eventually only one bent washer was holding it all together, but we were stuck firm. This stalemate could only be resolved by the application of much energy to the u-bolt via a lump hammer. This allowed for the action of gravity (acceleration being 9.8m/s squared, opposed by me pulling the other way to little effect). And it produced a hand injury caused by a shard of steel from the battered u-bolt.

Exactly why the bracket did not immediately break and send everything to the ground is something of a mystery. But it didn't and I had time to organise a more orderly collapse.

But, and surely this is the most important aspect, the 6m beam was undamaged.

I have learned that putting so much weight on those brackets is an issue when it comes to changing things around. They are OK when the entire system is assembled, but there is a risk in taking it apart. I have to take account of my physical limitations too. So the rotator and 6m beam will not be going back up on those brackets. It just gives me something else to think about. Ideally they would be below the quad bander, but that is not going to happen now.

If you hear me on 6m, it will be on the 1.2m long Mirfield Electronics quad band vertical! So, don't expect to hear much from me on 6m until I get something better sorted out.

73

Jim (feet back on the ground).

3 comments:

  1. Very useful, I'm looking at the same antenna, mostly for 4 and 2, but for occasional use on 6 to the local repeater. If you get it raised above the roofline, I'll be interested to know about the behaviour on 6.
    What is your coax? is the included PL259 a 10mm or 13mm deal? I thought all the threads were the same, interesting to know that they are not.
    G1WZM

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  2. I have replied to Clive direct.
    Just at the moment I cannot get this antenna to work properly, though of course that could be the coax or something at my end.
    We need an antenna like this and I am not giving up without trying everything. I have re-run the coax and I'll be back up the mast as soon as the weather permits.
    Oh dear.
    73
    Jim

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  3. Thank you for sharing, I too had been considering this quad band antenna for home use with my Icom 7100, but having read all your findings and that of your friend I will not be buying. If an improved version comes out in the future, then maybe.

    73 de Philip G0ISW

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