70 Mhz 4m LFA [Loop Fed Array]
(Also 70cm LFA see bottom of page)

To begin with I think this has been my favourite, single band, directional antenna. Not necessarily for the reasons you might think. In terms of my favourite 70 mhz antenna of all it has to be the Double Turnstile: some horizontal gain in all directions (except vertical and horizontal - it has to come from somewhere), no rotator required and bandwidth to burn; I can see why GB3BUX does so well on 70mhz with the same. However with the LFA I found it had more gain (albeit in one direction) and importantly a wide bandwidth due to its full wave loop. So it all depends what you need I guess. I like the structural integrity of the LFA also. My thanks go to G0KSC for providing the design and allowing use for amateur non-commercial puposes. What follows is simply my interpretation of another person's inspired idea. It works. Interestingly I also scaled the same antenna for 70cm without using any computer, and it worked fine too - yes 3 elements is not very much for 70cm, but it is compact.

The basic construction idea.

Waste not want not. I had a Double Turnstile which I constructed and used for a couple of years. It was solid in performance. But, you know how it goes, I decided a little more gain would be nice to the South-East, as I have a good take off in that direction... and I had not built an antenna for a while.. there was a buzz about the LFA... I reckoned I had all the materials so it should not cost me very much. So I got to work. First I dismantled the Double Turnstile. It was constructed from 1.25" tube and four 70mhz dipoles. The tube was long enough to form the boom for a 3 element LFA and the dipoles had enough aluminium (more or less) to form the Reflector, Director and Loop of an LFA. I have always been a big fan of folded dipoles in Yagi design. Very stable, mechanically strong and simple self-matching design. So when I saw the photographs of LFA it reminded me of an old Japanese folded dipole Yagi which I still have - Maspro 10 element 2m yagi. As well as many old school UHF TV Antenna driven elements made from flattened aluminium. In both of these it was the mechanical strength I liked.

Note the horizontal (in plane) folded dipole

In the Maspro (bought in 1995) the dipole was in the horizontal plane and (I always thought) stronger and less liable to wind load than the traditional folded dipole. Of course the LFA is based around a rectangular loop, but I thought it had similar benefits. I have other yagis which match with gamma/delta or whatever, but I always find these matching systems the weak point of the antenna. Water and wind will find a way to cause issues over time. The coax of the LFA is fixed directly to the element which is easy to seal. Win/win. If the antenna is any good that is - which it is! This page is based upon my notes seen below. I made these after taking the antenna down as I cannot find the original dimensions:



The measurements.

I had some problem in tracking these down. I read all of the information on the website which talks of tolerances of a mm etc. This put me off a little to begin with. Here was I with only scrap materials. It needn't have. The finished antenna (once tuned - see below) worked exceptionally well. Perhaps (I do not know) there could be further gain with more precision and better quality of materials, whatever, it out performed my 3 element commercial yagi even so. Now the measurements below are those as I found on the antenna when I took it down - for a further antenna test (see DK7ZB below).

Reflector 211cm
Director  189 cm

Driven 'loop' is: 176cm across by 33cm centre wide. (I had this way too wide to begin with and the SWR was very flat at 2:1) Later when I made it smaller the SWR achieved virtual 1:1. It was my eye expecting the traditional Reflector/Driven/Director to fall down in width gradually. The width of the driven loop is less than the director, as can be seen in the photgraph below. 176cm worked very well.

I will add a diagram below in time...

Construction

This is where a centre line along the full length of the boom is worthwhile getting correct. You do not want the elements as they attach to the boom to be out of plane with each other - doubtless why commercial versions use square boom. I find it very worthwhile to scribe/mark a line on the top and the bottom of the boom for the full length so that when I drill through for the dipole centres (I used 4 of these, more later) I drill from the centre line at the top and emerge out from the centre line below. This keeps each dipole centre square to the boom and in the same plane as the others.

I used the following dimensions: 35 cm from the centre of the reflector to the centre of the feed point dipole centre. 33 cm to the next dipole centre (which makes up the parallel section of the loop). 71cm to the director dipole centre.

The method employed was to drill through the boom. Attach dipole centres at four points. Only one of these was actually used for coax (via a coaxial coil balun) at the feedpoint of the driven element. The other 3 were simply bridged internally, using insulated wire as thick as would fit the dipole centre terminals. Once the half elements are attached the short makes them one single element, isolated from the boom. The use of the same dipole centre throughout keeps all of the elements the same distance from the boom.

The loop section was achieved with smaller diameter aluminium at the ends which slide inside the two loop elements to create the rectangle - trombone style. These slide in and out also to adjust SWR and clamp into postion. In my case a width of 176cm appeared optimum. The dipole centre nearest the reflector is the coax feedpoint, the other is bridged internally as above.

Balun.

The balun is ths simplest part of the whole design. I attached eyes to the end of the feed coax. These fitted inside the feedpoint dipole centre and were bolted into place withing a dipole centre, then sealed by the dipole centre cap (along with plenty of self-amalgamating tape 25mm wide, worth its weight in gold for any antenna builder, as well as liquid tape on the inner screws inside of the dipole centre). The coax behind this I wound four times around a standard aerosol can then secured with cable ties (zip ties) and removed the can leaving a loop in the coax which I fixed (cable ties again) to the boom. The farther end of the coax went to the shack. After tuning the coax was attached to the mast with cable ties. No joints, no chance of leaks - this is why I love the LFA.


See balun on top

SWR

As stated above at first I had the loop way too wide. I set it by eye to look like the classic yagi 'Xmas tree' profile. This is wrong. I eventually found the SWR was great at about 60mhz! It was then I realised my mistake and slid the loop ends further in until 1:1 (as far as the mass of the needle of my SWR meter was concerned anyway) was achieved. This antenna was up all summer and the following winter. It worked an absolute treat and will take whatever power you care to put through it. I only replaced it because I had need of 6m/4m/2m and I have only one pole. This was where the DK7ZB came in...

First Testing (Photograph)

Note the 4 dipole centre construction technique. Three of the centres are shorted internally apart from the coax feedpoint closest to the reflector - take care to keep these 'shorts' isolated from the bolts which hold the diploe centre to the boom, I just used insulated 20 amp wire. The coax coil balun is obscured by the pole - it is simply cable tied above the boom, see above.


70CM LFA

Just for fun I scaled down the antenna by 1/6.185 to produce the antenna below - sans balun. It works. How well I cannot tell as I only tried it indoors. However it demonstrated the same flat SWR over a wide frequency range.






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