A short 5 element LFA Yagi for 50MHz with a 4.7mtr boom

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The LFA design has a patent pending and all G0KSC designs are copyright. Any ham can build for their own use or those of a friend. Where profit or commerical gain exists, express permission of G0KSC must be sought.

There are 2 versions of this antenna below. One using 1/2 inch and 3/8 inch (12mm and 10mm) tube, the other 16mm 5/8 inch tube.

If you suffer with noise from living in the city, this is another low-noise antenna from G0KSC

This antenna has been designed in order to minimise the upward and downward lobes typically seen the the EL plane on Yagi antennas. By doing this, a drastic reduction in pick up of unwanted noise in City locations is the result. The double bonus is the likelihood  of reducing transmitted interference too. However, this version is not a model with optimum suppression which would see a reduction in forward gain of 1dB. If you suffer with very bad noise on 50MHz and have room for an antenna of this size, Email me for details of the very high suppression version.

This antenna can be built with Metric or Imperial sized tubing:

The 5el 4.7 mtr LFA on test at G0KSC


Martello Tower Group have the 5 element 4.7mtr 50MHz LFA and a 9el 4.5mtr 144MHz LFA

13mm 10mm and  1/2 inch 3/8inch element version

Performance

Gain: 10.93dBi @ 50.150MHz

F/B: 20.6dB @ 50.150MHz

Peak Gain: 11dBi

Peak F/B: 21.4dB

SWR: Below 1.1.1 from 50MHz to 50.500MHz

Antenna Dimensions

Spacing                      Half Element Size  in Metres              Element Diameter

Reflector                0                              1.488                                             10mm or 3/8 inch

DE1                      .290                          1.247                                             13mm or 1/2 inch

DE2                      .821                          1.247                                             13mm or 1/2 inch

D1                       1.477                         1.392                                             10mm or 3/8 inch

D2                       2.919                         1.3485                                           10mm or 3/8 inch

D3                       4.728                         1.293                                            10mm or 3/8  inch

The ends of DE1 and DE2 are connected together with 10mm or 3/8 inch tube. the Feed point is the centre of DE1.

16mm/ 5/8inch element version

Performance

Gain: 11dBi @ 50.150MHz

F/B: 22.37dB @ 50.150MHz

Peak Gain: 11.1dBi

Peak F/B: 25dB

SWR: Below 1.1.1 from 50MHz to 50.500MHz

Antenna Dimensions

Spacing                      Half Element Size  in Metres              Element Diameter

Reflector                0                              1.483                                             16mm or 5/8 inch

DE1                      .287                          1.258                                             16mm or 5/8 inch

DE2                      .828                          1.258                                             16mm or 5/8 inch

D1                       1.375                         1.384                                             16mm or 5/8 inch

D2                       3.027                         1.325                                             16mm or 5/8 inch

D3                       4.728                         1.3                                                 16mm or 5/8 inch

The ends of DE1 and DE2 are connected together with 13mm or 1/2 inch tube. the Feed point is the centre of DE1.

NOTE: SWR will not read as per the graph below if you use a package with mininec or nec2 the swr graph is correct and confirmed within nec4 and through building this antenna.

Any questions, please ask!

Justin G0KSC

I have exchanged mails and had conversations with hams that would like to build an LFA but the prospect of making a loop is a little challenging. Making the loop is really very simple and I have decided to detail the steps I take in order to make the loop in a few very simple steps.

First, an illustration by Rob, VK2GOM/G0MOH (thanks Rob) which shows the LFA arrangement. Note: pay careful attention to the design you have chosen as the feed point is within DE1 on some antennas and DE2 in others.

The tool for making the LFA loop can be purchased here

 Step 1

First of all, I treat the back and front of the loop as to separate elements (to start with) and place/mount them on the boom. Therefore, if I have a 6 element Yagi, without the loop ends joined, it looks like a 7 element Yagi. The picture below shows an antenna with the loop end piece laying on the floor next to the elements mounted to the boom. What we will now do is go back and bend another  piece of smaller diameter tubing to join the ends of the driven element and first director on the other side of the antenna in order to make this 7el Yagi, a loop fed 6 element Yagi.

Step 2

The elements on this antenna are all 1/2 inch or 12.7mm thick which is 18SWG. 3/8 inch or 9.6mm tubing fits nice and snug inside 1/2 inch so we will bend a piece of 3/8 inch tube to complete the loop. 3/8 inch is a nice size as a standard car brake pipe bender can be used to bend this tube cold. My one was purchased from Ebay for £3.99.

Make one bend in a length of 3.8 inch tube as per the photo below. Remove the pipe bender and insert the bent end into one side of the loop. In this case I will insert into the driven element section.

The photo below shows our length of tube with the bent end placed into the driven section of the loop. the other end is not yet bent. This is the part that makes the loop nice and simple to complete.

Step 3

Place the brake pipe bender back onto the 3/8 inch section of tube and move it along until the next element, the one we want to join to complete the loop, is sitting in the bend grove of the pipe bender (see photo below). Ensure that neither the back or front section of the loop is stressed and both are in their resting position. Apply a little pressure on the handles of the bender as if you were going to bend the tube but not too much. We want to just apply enough pressure so as the bender will not move on the tube as we remove the tube from the driven element section. the brender should remain in  its current position and should not move at all. 

While keeping pressure on the bender, remove the other side of the 3/8 inch tube from the rear drive section of the loop. Now complete the second bend in the tube.

Step 4

Now the second bend is done, you can now cut the end section to size, ready for final placement into the ends of the loop.

Step 5

Your done! now the section of 3/8 inch tube can slot perfectly into the ends of the loop and no tape measure or bend calculations have been needed! Make the choice of final connection of the loop carefully. While I do use stainless steel screws or nuts and bolts to hold these joints in place, the best option is a spot of aluminium at the 4 points where the loop meets the end section. This will ensure you will never, ever have an issue with water ingress or shifting SWR. This said, both my current 4/6M antennas are secured with stainless steel screws only and both perform now as they did when they were first installed.Furthermore, with such wide-band antennas as the LFA it is even less likely that any movement will ever be seen. However, I do like to point out the best way of doing everything we do!

The completed loop (below) with both sections in place. As you can see, the loop is straight and no bend exist where there should not be one!

The completed project, a 6el LFA Yagi for 50MHz. This one has a 1.5 inch square boom with a 1.5 inch secondary boom beneath the first. This ensures the 7.3 metre long boom will not sag and will not require any guying. This antenna has 12.3 dBi Gain and just over 23dB F/B.

Enjoy building your LFA and remember, PLEASE ask me if you get stuck or need help, no matter how silly the question sounds, I want to help you!

73

Justin G0KSC