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Author Topic: EFHW Again!!  (Read 1297 times)
KW9WK
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Posts: 199




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« on: December 03, 2017, 06:13:51 PM »

EFHW for 40m, (66' in the air) 9:1 balun at the feedpoint, ground side down to earth, with single 33' and 17' counterpoise wires along the ground - Anyone want to meet up some where on the LSB side and see how it plays?
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Lenny, KW9WK, formerly AC9RN, formerly KB2NYA....
KG5AHC
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Posts: 84




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« Reply #1 on: December 03, 2017, 07:49:39 PM »

Maybe sometime.

How did you come up with this arrangement? I am thinking that an End Fed Half wave would require a higher value transformer (such as 64:1 UNUN) to match the very high impedance that exists at each the end of a half-wave. 

I really don't mean to be picky, but I don't think you have actually  an EFHW.  Even though one leg is the approximate length of 1/2 wavelength for 40 meters, it looks to me a lot like an off-center dipole of sorts. However, if it gives you a good match (low swr around 50 ohms impedance), then it's probably ok no matter what we call it.. (it knows what it calls itself).

Regards,
Jeff KG5AHC
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KW9WK
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Posts: 199




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« Reply #2 on: December 05, 2017, 10:01:32 PM »

I came across some info somewhere, that was talking of the ins and outs of wire antennas - and suggested a 9:1 unun, which I employed. ( not a balun as in my op...) haven’t played a lot with it, I’ve been busy getting things working for digital on 20m - I will post more after playing some more!
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Lenny, KW9WK, formerly AC9RN, formerly KB2NYA....
WB6BYU
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« Reply #3 on: December 05, 2017, 10:12:15 PM »

Quote from: KW9WK

I came across some info somewhere, that was talking of the ins and outs of wire antennas - and suggested a 9:1 unun, which I employed. ( not a balun as in my op...)




Generally such a transformer is used for a "pseudo-random length wire", that is, one that is
intentional not resonant.  That means also not a multiple of 1/2 wavelength (which is
a resonant wire) because the un-un doesn't match the high impedance very well.  To maintain
a more reasonable SWR, the wire length is chosen to try to avoid a resonance in any band when
using a 9 : 1 un-un.

(I know, both are called "end-fed antennas", but they should be treated differently.)

The grounding system is not normal, but might work without too much radiation from the
"ground wire".  Ideally  you would put your radials right at the feedpoint, as the
wire currently running down to ground level is actually part of the antenna and will radiate.
(Not that the radials right at the feedpoint won't radiate also, but hopefully not as much.)
If the wire to ground was 1/4 wavelength it wouldn't act as a ground at all, but could actually
radiate a pretty good vertically polarized signal.


But, you have it up, so see how it works.
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NO9E
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Posts: 720




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« Reply #4 on: December 06, 2017, 06:04:27 AM »

With antenna tuners and short coax, the exact transformation and the lengths of the wire are not critical.

Take 15 and 28 feet. Take 17 and 22 feet. Probably you would not hear a difference.

What makes a difference is length and geometry. Efficiency plummets below a total of 1/4 wavelength. Higher is better. Vertical is good with good ground and bad with bad ground. Sloping is good in sloping direction.

Ignacy, NO9E
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KB1GMX
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« Reply #5 on: December 06, 2017, 09:02:52 AM »

While I agree with WB6BYU I'll point out a few things.

For other listening there are two forms of END FED antennas, resonant and non-resonant.
They are treated differently as the feed point impedance are radically different.  They
perform about the same for lengths  that are within about 20% but the matching network
is not interchangeable.  The last part is what important as we are trying to match a 50 ohm
radio and 50ohm coax to whatever the wire presents in as low a loss way as possible.

66 feet at 40M is a half wave and unless close to the ground it will have a high feed point
impedance.  Even if it were at the low end for feed point impedance (say at 20ft height) it
would be in the 1600 to 1800 ohm range if resonant. That impedance goes up with height
and peaks around 5000 ohms give or take.  A 1:9 transformer is 450 ohms expected load
for a mismatch of 3.5:1 or worse.  A 1:49 (1:7 turns ratio) transformer would be 2450 ohms
for a match and terminated with 1600 would give a VSWR of 1.53:1 or better.

* It begs the question why use a 1:9 transformer when its inadequate?
* Further why subject the ferrite and insulation in the transformer to stress
  from such a mismatch?
* Why run a high SWR on the COAX and add the need for an antenna matcher
  with its internal losses?

It also does one thing glossed over... The matching network has loss and its losses
can increase if the network is mis matched.  This means both the transformer and
the coax introduce mismatch loss when it is easily avoided.    For example we have
the resonant 66ft with a 1:9 at low height and this is a best case.  The transformer
translates the 1600 ohms to about 177 with added reactance as the transformer
was not designed to have enough inductance in the coils to work at that impedance
because it was designed for 50 ohms. We have a coax say 50 ft with a 3.5:1 SWR
and then a antenna tuner to correct that.  The antenna tuner is not 100% efficient
so there is loss there.

Take that same case and cut the wire in the middle and feed it with coax for a
likely SWR of 1.7:1 or lower.  With a suitable transformer of 1:36 or 1:49 we can
end feed the same wire with a match under 1.7:1 as well and maybe even save
on coax.  In either case the total losses in feeding the radiator are reduced and
the need for a tuner is potentially eliminated.


Allison
« Last Edit: December 06, 2017, 09:05:41 AM by KB1GMX » Logged
KD1I
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Posts: 397




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« Reply #6 on: December 06, 2017, 01:10:14 PM »

When the original poster said it was a 9:1 balun, we all assumed it was 9:1 impedance transformation.    Just suppose he meant a 9:1 turns ratio.
Then the impedance ratio is 81:1 and his 50 Ohm coax transforms to 4,050 Ohms.........   Just a thought. Maybe he could clarify this point.     Jim
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KU3X
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Posts: 435




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« Reply #7 on: December 07, 2017, 12:46:04 PM »

When the original poster said it was a 9:1 balun, we all assumed it was 9:1 impedance transformation.    Just suppose he meant a 9:1 turns ratio.
Then the impedance ratio is 81:1 and his 50 Ohm coax transforms to 4,050 Ohms.........   Just a thought. Maybe he could clarify this point.     Jim

Either way, it's the wrong balun. 64 to 1 is the unun of choice for an end fed halve way. Depending on the unun design, there are times to change to a 47 to 1 unun,
but never a 9 to 1 balun or unun, and I don't mean turns ratio.

One of the reasons to use a 9 to 1 is on a non resonant length of wire so the tuner sees a resonable impedance to work with. An example would be these 43 foot verticals. Yes, there are other uses for a 9 to 1 but not on a resonant end fed.

Barry
« Last Edit: December 07, 2017, 12:48:55 PM by KU3X » Logged
KW9WK
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Posts: 199




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« Reply #8 on: December 08, 2017, 05:01:45 PM »

Well, I have played with the 4:1 and 9:1 ununs, and I can get it to match up on 40m -  with the tuner in the shack - But yes, I understand that all I'm really doing is creating a low SWR and the transmitter is happy.  Reception isn't all that great. (Actually, I can get better S numbers on 40 with the cobweb built for 10 thru 20 meters.)   
 
The actual feedpoint is about 18" above ground, right next to the house - I have the two radials and an earth ground on the one side of the unun, and the 66' radiator in an inverted L configuration, about 25 feet high and about 30' on the horizontal.  Far from ideal. ( I think the antenna tuner has an easier time with the 9:1 unun in the circuit. )

My focus for the time being, will be on a halo for 6 meters, and putting up on the roof with the cobweb.  The cobweb is almost resonant on its bands, but is really tweaky - I don't like relying on the tuner in the shack to get it perfect, so when I have enough soda pop bottles saved up, I plan on a remote tuner up as close to the cobweb feedpoint / balun as possible. But I digress -
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Lenny, KW9WK, formerly AC9RN, formerly KB2NYA....
KG5AHC
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Posts: 84




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« Reply #9 on: December 10, 2017, 06:18:19 AM »

The actual feedpoint is about 18" above ground, right next to the house - I have the two radials and an earth ground on the one side of the unun, and the 66' radiator in an inverted L configuration, about 25 feet high and about 30' on the horizontal.  Far from ideal. ( I think the antenna tuner has an easier time with the 9:1 unun in the circuit. )

For this 40 meter single band antenna, I recommend that you consider buying or building a 64:1 or 49:1 toroidal transformer instead of the 9:1 unun.  Tune the antenna by adjusting its length.  Hopefully your 66 feet is just a bit longer than ideal and you can fold back the far end.  With this transformer and the correct wire length, you should not need radials or "counterpoise"...  or even a tuner (but its OK if it helps get that perfect match for the transceiver).

Alternatively you can use a tuned circuit coupler for the antenna wire. A bit more complex but achievable. I have not had much luck building one of these.   MFJ has a couple of portable manual "pocket" tuners that will handle up to 100 or 200 watts.  I have used my MFJ 971 portable tuner on occasion as a feed point coupler for a "random wire" antenna and even some of my tuned EFHW antennas-- to really cut down on any feed line losses due to reflected RF.

Cant guarantee that you will be happier with a different setup, but it's worth a try.

One more thought... I recently installed a Buxcomm line isolator on my feed line(s) very near the radio end of the coax to positively eliminate any common mode currents that might be occurring with any of my home-brew antennas.  I cant see common mode current, but I am betting it could be present on any coax feed line.

Good Luck and 73's
Jeff KG5AHC

 
 
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KC1BMD
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Posts: 612




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« Reply #10 on: December 10, 2017, 08:56:39 AM »

With this transformer and the correct wire length, you should not need radials or "counterpoise"...

I think it's a generally accepted fact that the EFHW antenna will always need a return current path (whatever you choose to call it). If one chooses to not physically provide one (and it doesn't need to be long for an EFHW, eg. 0.05 lambda could work), then the scientific laws governing such things will provide one for you (eg. coax shield, capacitance to ground, etc).
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KW9WK
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Posts: 199




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« Reply #11 on: December 10, 2017, 11:25:17 AM »

With this transformer and the correct wire length, you should not need radials or "counterpoise"...

I think it's a generally accepted fact that the EFHW antenna will always need a return current path (whatever you choose to call it). If one chooses to not physically provide one (and it doesn't need to be long for an EFHW, eg. 0.05 lambda could work), then the scientific laws governing such things will provide one for you (eg. coax shield, capacitance to ground, etc).

The radiator being 66' and the ground/radial/counterpoise, however we prefer to call it, being earth, 33' and 16' (give or take a few inches)  It tunes, but, (A big but)... Right now, the antenna is there, and I will play with it on and off.  Can't dent it is fun, I am learning, and I will be sharing what I learn as I go!!
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Lenny, KW9WK, formerly AC9RN, formerly KB2NYA....
KV7W
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Posts: 148




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« Reply #12 on: December 10, 2017, 12:12:50 PM »

Get the counter up off the "ground" about a foot or two. I've also found it beneficial to to use two counter wires off the ground in a V shape under the 66' wire. (Don't cut two wires, just strip the insulation from the middle of a long one.) Play around with spreading the counter wires between 25 and 45 degrees - and no, they don't need to be all that long.
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KW9WK
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Posts: 199




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« Reply #13 on: December 10, 2017, 06:18:00 PM »

Cool - I will play, for sure! The "counters" run along the ground near the foundation of the house - (I'm in a HOA sort of situation...)  But I will endeavor to do what I can, of course....
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Lenny, KW9WK, formerly AC9RN, formerly KB2NYA....
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