Nice new beam for 70MHz

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radiogareth
Posts: 392
Joined: Wed Jan 06, 2016 9:46 am

Nice new beam for 70MHz

Post by radiogareth » Wed Mar 18, 2020 3:17 pm

I've been working on this on and off as the weather permits. Its a 5 element LFA yagi for 70-71MHz. As described here:https://www.g0ksc.co.uk/70mhz-lfa-yagis ... m-lfa.html and chosen as the boom will just fit inside my radio truck (camper).
I built it exactly as per the published dimensions and the first pic shows the match - optimised for the DX end of 70. Justin (LFA boss) suggested slight shortening of the parasitic elements which I do today, second photo shows the end result. Best of both worlds?? Quoted gain 11dBi so I won't need my 300 watt amp for a while ;-)
Last pic shows it up in the air. 3-D printed 'knurled' captive nuts means that only an allen key is needed for on-site assembly.
Next project - 4 element for 6m, also on a boom that will fit inside the R/T-camper
LFA_4M.PNG
LFA_4M.PNG (3.26 KiB) Viewed 392 times
LFAFINAL.PNG
LFAFINAL.PNG (3.1 KiB) Viewed 392 times
Also of possible interest is a tri-band MOXON (6,4 and 2) based on a design in SPRAT 2018 (autumn) which uses coaxial traps and might be handy for scanning around. I've built it from overflow tubing and glassfibre electricians rods (4mm diameter) for flat-packing. Various bits of 3-D printed bits help with holding it all together.
MOXNLAST.PNG
MOXNLAST.PNG (3.38 KiB) Viewed 392 times
2020-02-11 15.58.55.jpg
2020-02-11 15.58.55.jpg (1.16 MiB) Viewed 360 times


Gareth, G4XAT
Last edited by radiogareth on Wed Mar 18, 2020 7:48 pm, edited 2 times in total.

M0YDH
Posts: 184
Joined: Sat Dec 31, 2016 5:59 pm

Re: Nice new beam for 70MHz

Post by M0YDH » Wed Mar 18, 2020 6:59 pm

That looks interesting Gareth. It would be worth adding a link to this post at https://wiki.batc.org.uk/71_MHz#Antennas

I made the 4m cubical quad seen in Radcom earlier this year. The support tubes are 25mm conduit using intersection boxes. I recalulated for 71 MHz. The telescopic diagonal tubes are worth the extra effort because the square of wire cut to the calculated length doesn't account for bridging the tube diameters at each corner. And you can take up the slack. I recycled the boom and pole from our former TV aerial. After much phaffing about I found a narrow bandwidth low VSWR match at 71MHz.

I hope we're allowed out on Easter weekend for ATV activity days. :?

Stay safe and well

David M0YDH
Attachments
Narrow bandwidth match was the best I could get after 3 sessions
Narrow bandwidth match was the best I could get after 3 sessions
ERePF9RX0AIwKzy.jpg (367.99 KiB) Viewed 370 times
Cubical quad
Cubical quad
ERePCKgWsAMGyoi.jpg (121.28 KiB) Viewed 370 times

radiogareth
Posts: 392
Joined: Wed Jan 06, 2016 9:46 am

Re: Nice new beam for 70MHz

Post by radiogareth » Wed Mar 18, 2020 7:59 pm

Its good to experiment :-) MMANA-GAL (now that I've found I can enter the wires in a GUI of sorts) is great for testing out stuff. I've also completed a full-wave dipole for 2m (talkback) that's better than a 7/8th mounted above it, and an interesting this called a zig-zag. This delivers nearly all-round gain (6dBd) and although vertically mounted is horizontally polarized.
Pics attached. The zig-zag really needs shorter elements which I'm giving thought to. The electricians rods made for easy assembly as the ends are threaded and did the 90 degree joins nice and simple. Just a shade too long with insulated wire, might try with ECC instead.
2020-03-02 15.06.30.jpg
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This is the zig-zag 'thing'.....
2020-03-02 15.06.30.jpg
2020-03-02 15.06.30.jpg (386.3 KiB) Viewed 357 times
And this shows the 7/8th above the full-wave, full wave gave 2 S points UP on GB3VHF compared to the 7/8 (both vertical polarization and beacon horizontal AFAIK)
Plot of the full-wave....
Full-wave.PNG
Full-wave.PNG (18.1 KiB) Viewed 356 times
My favorite TX spot is by private arrangement so I should be able to 'go out'. Subject to farmer being happy with it.

FIngers crossed :-)

Gareth
Attachments
2020-03-04 14.48.12.jpg
2020-03-04 14.48.12.jpg (92.99 KiB) Viewed 357 times

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