Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

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Stronty
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Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Stronty »

Inspired by @Buick Mackane I decided to make a Multi-Band EFHW 80-10m 49:1 Transformer and a choke.

80>10 EFHW Transformer 49:1
  • I already had 2x FT240-43's (Fairrite) I bought from Enigma shop about 10yrs ago.
    250g X 1.8mm Brocott coated wire (that's plenty)
    2 X IP56 rated enclosure 100x100x50 from Screwfix. (second enclosure for other project)
    1 X 100pF 6kv capacitor (blue) from Tekkikid on eBay.
    50m X of poly coated flex weave, should not need more than 44m or slightly less.
    2 X SO239 UHF Female Chassis Socket Square Flange Panel Mount.
    10 X 2mm Duplex Wire clamps
    4 X M6x12mm Eyebolt Stainless steel, used 1.
Various other bits of hardware like wingnuts, washers, lock washers, nylon lock nuts & electrical ring terminal connectors.


Having read a fair bit about Multiband EFHW's since our Buick reminded me they are a thing on both his threads,

£12 Homebrew EFHW

EFHW Tuner / LED SWR Bridge

I resurrected a project I meant to start almost a decade ago.

I'd intended to make a common mode choke years ago, bought all the bits and then lost interest in radio. I chose to make one based on reading G3TXQ (silent key now) site back then but read more this time round and tweaked a little.

Common Mode Choke
My reading lead me to a 9 turns of RG142 over 2 X FT240-31.
  • I already had 3 X FT240-31 again from Enigma shop, I only used 2 though.
    4m X RG142 from Engima shop 10 yrs ago (doubled in price now)
    1 X IP56 Enclosure.
    2 X SO239's
    1 X M6x12mm Eyebolt Stainless steel.
Various hardware for securing M3.5 holed SO239 sockets, locking washers etc.

Also worth having some cable tie anchors and good grippy cable ties.

BTW pulling RG142 tight is a complete bugger.

So pics attached for perusal.

And no I haven't deployed it yet as I need help sticking up the 44m or so antenna but when I do I'll let you know.

Thanks for the inspiration Buick.
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Buick Mackane »

Nice work stronty.

Before deploying it it's best to bench test it, You can do this by connecting a 2.5k resistor across the output of the 49/1 transformer and measuring the SWR, Your 2 x FT-240-43 cores should be good for a few hundred watts. My homebase EFHW uses a single FT-140-43 As i only run a max of 50 watts. (2M0 Callsign)

For a counterpoise i use 7 foot of wire which is the recommended length for a 40 - 10m EFHW, I forget the exact maths but the counterpoise should be a fraction of a wavelength at the lowest band you intend to work,

Or you can install a common mode choke at the point where the coax enters your shack and let the coax braid function as your counterpoise.


Mine seems to work well with no RF / Common mode issues. Let me know how you get on.,
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by ch25 »

Any VNA measurement screenshots?
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Stronty »

Thanks Buick I will need to test and will use the method you describe.

Building something carefully does not guarantee it will work as intended never mind working in my own circumstances.

Good point ch25!
I have been looking at the various nano vna’s on eBay and Amazon. Anything from £45 to £95 ranging in screen size etc.
I’m not sure I’ll get heavy into radio again, I fancied making use of parts I bought yrs ago and maybe getting into it again, we’ll see. Spaffing even £45 on a bit of kit I’m not likely to use much isn’t really on my radar BUT I DO want to know if what I have built works so maybe I will.

I should just do it the old fashion way and do some measuring via my radio but the nano vna certainly reduces that pain from a full day down to a matter of minutes.
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Stronty »

Only took 4 months!

I have spaffed on a nano-vna and pigtail option for SO-239/PL-259.

Got some 2.5k resistors winging their way, not sure how "non-inductive" they are but we'll see.

I should know by next weekend if what I built works hopefully.
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Stronty »

So NanoVNA's are an interesting bit of kit.

So as per Buicks suggestion, connected a 2.4k resistor should really be a 2450 Ohm across the ground and the Antenna wire connections.

I'd say that looks ok, maybe not brilliant at the 10m band but that's not where I built it for.

Playing around with compressing & spreading the first 7 coils does improve things higher up though.

49-1.jpg
8010EFHW.jpg
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Buick Mackane »

Nice one stronty.

Mine is resonant higher up on 10m than i would like, But perfect on the lower bands, Around 1.1 to 1.3 VSWR. Just bought 20m of polyweave wire to replace my speaker wire when it eventually breaks but holding up ok so far.

steel washing line is a fraction of the price of polyweave antenna wire and looks exactly the same, Never tried it but curious as to whether it would work or not
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Buick Mackane »

Have you got this up in the air yet stronty ?
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

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Buick Mackane wrote: 27 Nov 2022, 14:48 Have you got this up in the air yet stronty ?
Was my plan for yesterday, as was an ok day given the recent weather. Sadly other duties took over but if it stays mild and damp I'm sure I'll manage it before Xmas.

Nano VNA has been sent back though, another one will be purchased. Looks like I got someone's return, was a little suspicious in that it looked a little used. but there was some things about config retention or even changing config without actaully changing it after reboots that made me a little suspicious about it.

There is lots of great help and information about the various NanoVNA's out there, i found this group had many files and topics of interest. https://groups.io/g/nanovna-users

As soon as I have it out and a rig attached I'll be sure to update the thread.
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Re: Multi-Band 80>10 EFHW Transformer and a Common Mode Choke.

Post by Buick Mackane »

I'm sure it'll work fine with a bit of fettling to get the SWR where you want it. I use a cheapo sark-100 analyser and TBH it works fine and dandy despite what some of the online reviews say.

I was toying with the idea of building another EFHW Using the bigger FT-240 Core but no point really as the one i'm using works as intended, But that doesn't mean i won't build another one just for the hell of it :)
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