40M EFHW & 3D print...
 
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40M EFHW & 3D printing

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Jed Baer
(@kd0ymg)
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Joined: 12 months ago
Posts: 58
Topic starter  

I've started on yet another antenna project, this time a 40M EFHW. There's more to these than it seems, especially when you start digging. I've been reading threads on QRZ, and the discussion is often quite lively, which doesn't always translate into "useful", but there's some folks there, especially WA7ARK, who seem to know their stuff.

I've wound my toroid as an autotransformer. I might change this a little, or just solder on with what I have.

My main questions have to do with tuning the thing. Yes, the common wisdom is to trim to length for the lowest band, and from what I've seen on YouTube, this often has pretty good results, but the harmonics aren't actually perfectly aligned, and so true multi-band, no tuner, operation isn't what you get. This gets off into the question of using "compensation", which is a new concept for me. There's more than one way to do it, and you'll see compensation coils on a couple of commercial units, such as the Chameleon LEFS.

That Chameleon, BTW, seems crazy good. It uses a coil as a compensation unit, and no counterpoise.

There's seemingly conflicting information out there, such as on QRZ the consensus seems to be to either use no choke on the coax, or choke it some ways away from the feedpoint - WA7ARK recommends .07 wavelength, which is about 9' on 40M. But Mike - K8MRD shows using the LEFS with a choke right up there at SO-239.

And I wonder, since I use a counterpoise to tune the homebrew Buddistick, why not use one on an EFHW as well. I think it'd work well with choking the coax right up there at the feed point.

I'm also thinking of simplifying the frame I'm looking at making to house the toroid, etc. Maybe it's fine as is. Could be improved? I'm designing it in TinkerCAD, and a friend of mine will print it out on his Prusa.

If anyone wants the STL file, shoot me a message.

EFHW label

   
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Scott AK6Q
(@ak6q)
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Joined: 3 years ago
Posts: 61
 

Howdy Jed!    Hey, a few tidbits that I have learned along the way with EFHW antennas.   

We need to look at some items first, what power are you going to be putting into this antenna....10-15w or 100W or more???

Is your core sized correctly for that power. 

What did you build the autotransformer to, a 9:1 or something else? 

On a resonate EFHW you will want to build the transformer as a 9:1, with this you can make it for 40M and you will get harmonic use on 20 and probably one of the higher bands like 10 or 12.

IF you want to build it as a non-resonate antenna (random wire) then you will need to up the winding and make the transformer a 49 or 64:1 ratio. 

On an End Fed antenna there is no counter poise per-se as when you add a counterpoise it become a Dipole.  Now on the flip side of this is, the coax shield becomes the counterpoise of the antenna so you will not want to choke the feed point of the antenna.  You will need to at least 9-10' of coax. 

I am real interested in seeing what you come up with. 

Elecraft KX2 KX3 K3, Icom IC-705 Yaesu FTD3, VX8


   
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AB4BA
(@ab4ba)
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Joined: 4 years ago
Posts: 29
 

Scott, are the baluns/transformers reversed?  Shouldn't if be a 9:1 for non-resonant long wire, 49:1 for a resonant EFHW?

AB4BA


   
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Jeff Karpinski
(@webmaster)
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Joined: 4 years ago
Posts: 120
 

Posted by: @ab4ba

Scott, are the baluns/transformers reversed?  Shouldn't if be a 9:1 for non-resonant long wire, 49:1 for a resonant EFHW?

AB4BA

 

Indeed.

Additionally, while a counterpoise isn't exactly required for an EFHW, a short one may help, or at least insure the feed coax runs along the ground for a few feet. For a random wire antenna, a counterpoise is required and its optimal length varies depending on operating band, ground conditions, etc.

Good info on "random" wire lengths can be found here: https://udel.edu/~mm/ham/randomWire/

As one can see, they're anything but random as only specific lengths will actually work.

 


   
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Jed Baer
(@kd0ymg)
Member
Joined: 12 months ago
Posts: 58
Topic starter  

Posted by: @ak6q

Howdy Jed!    Hey, a few tidbits that I have learned along the way with EFHW antennas.   

We need to look at some items first, what power are you going to be putting into this antenna....10-15w or 100W or more???

Is your core sized correctly for that power. 

What did you build the autotransformer to, a 9:1 or something else? 

On a resonate EFHW you will want to build the transformer as a 9:1, with this you can make it for 40M and you will get harmonic use on 20 and probably one of the higher bands like 10 or 12.

IF you want to build it as a non-resonate antenna (random wire) then you will need to up the winding and make the transformer a 49 or 64:1 ratio. 

On an End Fed antenna there is no counter poise per-se as when you add a counterpoise it become a Dipole.  Now on the flip side of this is, the coax shield becomes the counterpoise of the antenna so you will not want to choke the feed point of the antenna.  You will need to at least 9-10' of coax. 

I am real interested in seeing what you come up with. 

I'm figuring no more than 25W. There's a lot of suggestions out there for using 2 or 3 toroids, stacked, for more power. Also, the diameter has some effect there as well. The commercial units seem mostly to use an FT240-43 at the least, except for the QRP models. My toroid is an FT140-43. The 1KV capacitor is limiting as well.

The autotransformer is 49:1. Initial testing with a 2.7K resistor shows more transformation than that - more like 100:1 at 20M, better at 40M, but behavior with a real RF load might be different. And the actual feedpoint impedance could certainly be higher - it depends on how the antenna is deployed - I don't know how much variance to expect here.

The end-fed antennas are really dipoles anyway, though it isn't apparent. Simply put, there has to be at least one other element, even if it's "implicit", since the RF needs something. The term "monopole" is only physically descriptive. There's some debate on this point, I guess. Dave Casler refers to the EFHW as a dipole, and when a counterpoise is added, it becomes a tri-pole. I'm still working on the concept of whether it's a viable concept to tune using varying length of counterpoise.

Mostly, at this point, I'm pondering whether to vastly simplify my 3D printed frame. The turnaround time on test prints for me, since I'm relying on a friend, is measured in days / weeks, so feel pretty handicapped there. I'm also reluctant to have him print something that takes 8 hours, only to chuck it in favor of a revised design.

 


   
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