Microlight 40M OCF portable antenna

[Lamewolf]

Going to go ahead and build my microlight off center fed dipole to use with my Icom 703 and its built in ATU.  The balun will be a 3 core design using three FT50-43 cores.  Two of them will be wound as a 2 core guanella current balun and the third will form a 1:1 choke and all be mounted on a fiberglass board for the center insulator and then seal with liquid tape, and use some very flexible #20 rubber insulated wire a friend gave me for the antenna sections.  It will be fed through 33' of RG316 coax that I ordered and terminated into a BNC connector.  Using the BNC so it can also be used with a couple of small QRP homebrew tuners that I use with my Yaesu FT-817.  The balun will probably handle around 20 watts at most but that's ok as the 703 is a 10 watt rig and the 817 is a 5 watt rig.  But I know the ATU in the 703 will handle it just fine on all bands 10 thru 40 if it does need an ATU.  I will initially make the antenn for 10 thru 40 meters but may also make up a set of extender wires to make it work also on 60 & 80 meters when needed.  The antenna will be supported for portable use by my 31' Jackite telescopic pole as an inverted V.  Should prove out to work quite well even at 10 watts !

[cockpitbob]

Sounds nice.  I don't have a telescoping pole and keep thinking I should get one, but living in New England we have more trees than we know what to do with, so my slingshot has always been enough.  How stiff is the pole?  Does it stand pretty erect holding up an inverted-V?

Post lots of pics please.  We like pics  ;D

[Lamewolf]

The pole won't hold anything real heavy but is made to hold a wind sock like you see at airports and on beaches.  A lightweight doublet made from #20 speaker wire and fed with 300 ohm TV twinlead is the heaviest antenna I've ever hung on it and it did ok but with a little bow in the top section.  The best use I've found for it is to hold the vertical section of an EFHW when setup as an inverted L or just a vertical wire the same length of the pole fed at the bottom with a tuner or a unun/tuner combo  It does need some sort of mount at the base to support it with and I have a steel plate with a tube on it that the pole slides into and I can use gutter spikes to pin it to the ground or I can simply park one tire of my suv on it to anchor it.  One ham I saw used a "BIG" screwdriver that would fit up inside the base of the pole and he just drove that in the ground and slid the pole down over its handle.  They are handy to have and collapse down to 44 inches for travel so it could be tied on the side of a backpack if needed.  They also make a good fishing pole ! :)

[gil]

Nice project, similar to what I did with my 135ft Windom. My dual core Guanella BALUN works great. You might not even need the choke but it won't hurt. Mine is resonant on 80/40/20/10m. I have not tried it with a tuner yet. Please do post pics!

Gil

[cockpitbob]

Gil, isn't it also resonant on 15?

[gil]

It should be, but surprisingly no.. I will test more soon.. I think the SWR was high on 15 but not overly so.. I can't find my notes.. It will undoubtedly work fine with a tuner. So far the antenna seems pretty efficient. Still no contacts with the US though, depressing.. I am looking forward to Lamewolf's report..

Gil

[Lamewolf]

The swr on 15M on an 80m OCF is usually pretty high unless you do and sever off center split on the order of 17% & 83% then you get a lot of common mode current due to the severe imbalance but that's what the choke is for.  But with the old 1/3 to 1/4 offset, 15M is usually unusable without a tuner.  So is 30 meters.  My 80 meter OCF is usable on 80, 40, 20, 17, 12, 10 and 6 meters but has the old style split of about 45' and 90'.

[Lamewolf]

BTW, I meant to add that to get 15 meters on an 80M OCF you have a couple of options.  One is to use a voltage balun to feed a standard 45/90 split antenna and the voltage balun will allow the feedline to radiate but put a good 1:1 choke 22' below the voltage balun to make that portion of the feedline resonant on 15 meters - this is called a "Carolina Windom". I do not like a radiating feedline if I can help it but it does work.  The other option is to use a different offset, and with something like a 25' - 109' offset, 15 meters will work too but does cause and imbalance that would normally cause feedline radiation, but that can be cured with a good 1:1 guanella current balun connected between the coax and the 4:1 balun and the 4:1 and 1:1 can be built into the same box if you wish.  This will allow 15 meter operation but also assures that the antenna is radiating the signal and not your coax !  And that's what I plan on with my microlight 40 OCF.  It will use lengths around 17.4' and 55.6' and should cover 40, 20, 15, & 10 meters without a tuner plus 12, 17, & 6 meters with a tuner but 12, 17, and 6 meters should have a low enough swr to still be fairly efficient and not too much loss on it.  As long as the swr is 3:1 or under, the losses aren't really all that high and the antenna will still work well when using a tuner.  And the super thin RG316 coax (about the same diameter of RG174) is still fairly low loss and from what I've read will handle a lot of power !  But I'm using it to save weight since I will only be running around 10 watts through it.

[gil]

I plugged in my KX2 tonight and my Windom is indeed resonant on 15m. It does however have a high SWR on 20m.. Good on 80/40.. See the thread about it for measurements.. What matters more to me here is 80/40 anyway. I can use a tuner on 20..

Gil

[Lamewolf]

It should be, but surprisingly no.. I will test more soon.. I think the SWR was high on 15 but not overly so.. I can't find my notes.. It will undoubtedly work fine with a tuner. So far the antenna seems pretty efficient. Still no contacts with the US though, depressing.. I am looking forward to Lamewolf's report..

Gil

Gil,
I looked at my graph on my 80M OCF and the SWR on 15 was around 5:1 which is fairly normal but it was a flat 5:1 across the band.

[Lamewolf]

What was the title of that thread Gil ?

[gil]

Mine is 44+89ft.
Gil

[gil]

The thread is "my new antenna situation."

Sent from my A1-830 using Tapatalk

[Lamewolf]

Mine is 44+89ft.
Gil

My 80M OCF is also 44' & 89' with a dual core guanella current balun wound on FT240-61 cores.  But I didn't get so lucky on 15M where the swr is 5:1 across the band.

[gil]

I think I used the same cores, weird. My high SWR is on 20m.. Has to be environmental. The last third of the antenna slopes down after going over a tree branch..

Gil