Balun for a end fed random wire?

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paul
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Joined: Wed Aug 03, 2022 12:56 pm

Balun for a end fed random wire?

Post by paul »

I regularly just use a random wire antenna to boost my portable shortwave radio listening. But I have read that a 9:1 balun may improve things. I think I will have a go at making one. But I wondered if anyone here had any advice on having actually tried one in this arrangement. Is it worth it?
13dka
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Re: Balun for a end fed random wire?

Post by 13dka »

A balun is for connecting balanced antennas to an unbalanced feedline. For an end-fed wire you'd want an unun. It won't change the general characteristics of random wires, what you may or may not gain from it in reception (which depends entirely on your individual location/situation) is maybe a little less noise pickup, which is a pretty unspecific statement floating around on the internet.
paul
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Joined: Wed Aug 03, 2022 12:56 pm

Re: Balun for a end fed random wire?

Post by paul »

Thanks, 13dka - that is really helpful.

I think what threw me is that the instructions for making baluns and ununs is the same! They seem to say the only difference is how they are wired up at the end.

So I will try and make a 9:1 unun and see if it makes any improvement to the noise.
Egil - LA2PJ
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Re: Balun for a end fed random wire?

Post by Egil - LA2PJ »

Paul,
When I moved to my present home I had excactly the same problems.
After a lot of trial and error whithout any improvements in the noise situation, one of the oldtimers recommended a "Common Mode Choke".
He even built it for me. That got rid of almost all the noise, and still works fine with my 26 meters of longwire after 18 years.

Good luck!
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Andrew (grayhat)
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Re: Balun for a end fed random wire?

Post by Andrew (grayhat) »

Regarding the impedance transformer, an easy to build one is shown here

http://www.kk5jy.net/LoG/

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it's a "galvanic transformer" (that is, the antenna is electrically isolated from the feedline) wound on a #73 binocular core, as for the transformation ratio, that's easy to calculate if one keeps in mind that the turns ratio is the square root of the transformation ratio, so for example, willing to wind a 9:1 we calculate the square root of 9 which as you know is 3, now 3 is the turns ratio, so if we wind 6 turn on one side and then 6/3 = 2 turns on the other side, we obtain a 9:1 ratio for the transformer, at that point we just connect the ends of the 6 turns winding to our antenna (in this case, having an endfed, one end goes to the antenna and the other to a counterpoise wire); done that we will also need a common mode choke, this is needed to avoid that the coax feedline (external braid), due to the common mode currents, could act as an antenna and pick up lots of noise, such a choke may be easily wound using a #43 toroidal core, an FT140-43 will fit, just wind 17 or 18 turns of coax over the toroid as shown here

https://www.dj0ip.de/rf-cmc-chokes/diff ... lla-choke/

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and you'll have your choke, you may place it inside the same box hosting the transformer or in a separate box, one side of the choke goes to the transformer (the ends of the 2 turns winding in the above example) and the other goes to the coax connector where the coaxial feedline coming from the station reaches the antenna; willing to use a "belt and suspenders" approach, one may wind a second choke and place it right before the connection between the coax and the receiver

As a note, a "random" (endfed) isn't exactly the quietest antenna when it comes to receive signals, if you have room you may either consider the "Loop on Ground", the "SRL" or the "SULA" which will offer better SNR than the endfed
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Andrew (grayhat)
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Re: Balun for a end fed random wire?

Post by Andrew (grayhat) »

Forgot, the "9:1 transformer" isn't a magical object, the transformation ratio may be appropriate for some antennas but totally unappropriate for others, keep in mind that if the antenna wire is near (or below) 1/4 wave at a given frequency, the impedance will be very low so dividing that by 9 won't be appropriate, the same goes if the antenna wire is 1/2 wave or a multiple of 1/2 wave, in such a case the impedance will be in the 3KOhm range and the transformer won't give a good match (in such a case we'd better go for a 49:1), now you may wonder why people uses the "9:1" with a so-called "random wire" antenna, well, this is because such an antenna, used on the Ham (HF) bands can be designed to be longer than 1/4 wave at the lowest desired frequency and far away from 1/2 wave and its multiples on all the frequency such an antenna will be used on, with such a simple approach, the resulting antenna will present an impedance (on the Ham HF bands) ranging more or less between 300 and 600 Ohms so the 9:1 will achieve a pretty good match with the coax feeder, now, while having a good impedance match is important for TX it isn't so important for RX, although desirable, but the choice of the transformer is still something which needs to be carefully evaluated, the above being said if you want to read more about the "not so random" antenna, I suggest reading the informations found here

https://udel.edu/~mm/ham/randomWire/

Oh and... please don't call any "end fed" antenna a "longwire"; nothing to worry, but it's a matter of understanding, an EFHW is an end fed with a length of 1/2 wave on the lowest frequency and which can be used on the harmonics, for example an EFHW cut for 7MHz that is the 40m band, could be used (with a 49:1 transformer) on the 20 and 10 meters bands too, a "random" is as we discussed above a NON resonant antenna cut to avoid very low or very high feedpoint impedance, so that a 9:1 transformer may give an acceptable match, and then there's the "longwire" the latter is an endfed wire with a length of 1 full wave or multiples of it at the lowest desired frequency, so as you can see they are all "end fed" antennas but they're quite different each other (and then I omitted the beverage antenna :D)
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