Random wire antenna

[Andrew (grayhat)]

Let me start by saying that, what follows, is just a bunch of personal notes about the so-called "random" wire antenna, the idea of posting this came after reading some comments on QRPer.com

For the folks interested in building their own "random" wire antenna and not willing to deal with calculating a trasformer winding and so on, the following links will just show what's needed to put together a random wire antenna

* Optimal lengths for the antenna wire
https://udel.edu/~mm/ham/randomWire/

* 9:1 UnUn building instructions
https://vk6ysf.com/unun_9-1_v3.htm

* 1:1 common mode choke (optional but won't hurt)
https://vk6ysf.com/balun_choke_balun_hf_reisert.htm

basically, all you'll need will be selecting a length of wire for your antenna from the first link above, adding some wire for the counterpoise (length not critical), and then building a 9:1 UnUn to feed the antenna using coax and, optionally, a choke to improve it a bit (it helps reducing common mode currents and, in turn, improving both radiation and reception); if you don't use a counterpoise, the coax braid will act as counterpoise, in such a case if using a choke, it should be placed near the transceiver, and the coax (braid) running between the choke and the antenna (the 9:1) will act as part of the antenna, both when transmitting and when receiving (so for a "home" installation it would be better going for a counterpoise, otherwise the coax will radiate and disturb other appliance and will also pick up whatever noise it "meets" along its path)... was I saying... oh yes, if you don't use a counterpoise, ok, if instead you use a counterpoise, the choke may be either placed inside the same box hosting the 9:1 or in a separate box, for portable use, this second option may be better since it will allow more flexibility

Anyhow, once you'll have your antenna wire, counterpoise, 9:1 UnUn and the optional choke, you'll be ready to put your (not so) "random" antenna on the air, setting it up is not a "science" and when it comes to portable operations, one may setup the antenna as the location permits, so I'll just add a note; setting up the wire as an inverted-V or inverted-L (or even a "vertical") means having the feedpoint near the ground, this will increase losses but sometimes will be the only viable setup, otherwise, one may try putting up the antenna more or less horizontal, in such a case keeping the feedpoint as high as possible and leaving the antenna wire and counterpoise somewhat horizontal or sloping down will be a good idea

Now, to conclude, for the ones which want to fathom the details related to winding the 9:1, the choice of the core material and so on, here's a document which shows all the details and calculations involved

https://cromwell-intl.com/radio/9-1-unun/

while here's a quick reference page showing the characteristics of various materials

http://toroids.info/FT140-43.php

again, I'm NOT pretending to teach you stuff, the above is just a quick "digest" which may be useful in case one will decide to try a random and will want to try building it, instead of buying, all in all it isn't all that complex, is it ?

[N8TGQ]

Great synopsis on the random wire antenna, Andrew!

One thing I would add, for people finding this having no previous experience with random wires, is that this antenna requires a "tuner", or matching device. Many radios have one built-in these days, but many others don't.

Random wires are designed to be non-resonant on the bands you choose. The 9:1 unun just brings the impedance to a value that the tuner can easily match.

Thanks, Andrew!

Rick N8TGQ

[Andrew (grayhat)]

Great synopsis on the random wire antenna, Andrew!

One thing I would add, for people finding this having no previous experience with random wires, is that this antenna requires a "tuner", or matching device. Many radios have one built-in these days, but many others don't.

Random wires are designed to be non-resonant on the bands you choose. The 9:1 unun just brings the impedance to a value that the tuner can easily match.

Thanks, Andrew!

Rick N8TGQ

Rick, first of all, thanks for your nice words and I sincerely hope my brief notes won't be taken as some kind of "definitive something", they're just notes

That said, thank you for the reminder, yes, using a random wire requires a matching box, be that embedded inside the TRX or external, that's due to the fact that, even if the 9:1 helps bringing the impedance (R+X) at HF bands down to reasonable levels, the resulting mismatch is too high to make a transceiver happy, so an "ATU" is needed, now willing to build one, there are several designs available online, but one which I found pretty nice is the so-called "Fri Match" designed by PA0FRI

https://pa0fri.home.xs4all.nl/ATU/FRIma ... tcheng.htm

it can be built for QRP as well as for QRO, doesn't require switching (which makes it ideal even for a remote tuner unit !) and uses just a couple of (dual gang) capacitor, plus the beauty of the unit is that to adjust it, you just tune the parallel capacitors for max noise in RX and, done that, you pass to TX and perform the fine adjustments to find the SWR dip, it's really easy to use

[SkipRD]

Great summary, Andrew. Thanks.

For those interested in an off-the-shelf tuner (rather than building one from an online design), I tune up my Tufteln random wire antenna (the same antenna featured by Thomas in his activation of Cap Tourmente National Wildlife Area (VE-0012)) using a little MFJ-9201 QRP "pocket tuner." It tunes that antenna throughout 10-80 meters and seems to be reasonably efficient. It's a pretty simple little unit (one inductor and two variable capacitors) and weighs only 6 ounces. Not as much fun as building a unit, but an inexpensive and easy option.

73 Skip K4EAK

[Andrew (grayhat)]

Great summary, Andrew. Thanks.

For those interested in an off-the-shelf tuner (rather than building one from an online design), I tune up my Tufteln random wire antenna (the same antenna featured by Thomas in his activation of Cap Tourmente National Wildlife Area (VE-0012)) using a little MFJ-9201 QRP "pocket tuner." It tunes that antenna throughout 10-80 meters and seems to be reasonably efficient. It's a pretty simple little unit (one inductor and two variable capacitors) and weighs only 6 ounces. Not as much fun as building a unit, but an inexpensive and easy option.

73 Skip K4EAK

Hi Skip, you're right, I think I was unclear; by suggesting the ATU (which is needed for a "random") I probably gave the impression that one should build it, instead I was following my initial post and adding infos about diy matching units, but nothing forbids to buy a tuner instead

[Brian - K3ES]

Can anybody offer thoughts on the use of dedicated counterpoise wires with 9:1 Random Wire antennas for QRP operations?

I have a Packtenna 9:1 with 71 ft radiator that I have used with some success with only the 15 ft coax feedline for a counterpoise. I also have a Tufteln 9:1 with 35 ft radiator and 17 ft counterpoise wire that I have used with some success. Both antennas tune easily on some bands, but seem finicky on others. I have not yet experimented with different counterpoise wires due to the added complexity for backpack-portable operations.

Transceivers are a KX2 with internal tuner and a TX-500 with a ZM-2 tuner.

Might counterpoise wires of different lengths help with tuning on problem bands?

73 de Brian - K3ES

PS. I guess I should say that I have had no issues with "RF in the shack".

[Andrew (grayhat)]

Can anybody offer thoughts on the use of dedicated counterpoise wires with 9:1 Random Wire antennas for QRP operations?

I have a Packtenna 9:1 with 71 ft radiator that I have used with some success with only the 15 ft coax feedline for a counterpoise. I also have a Tufteln 9:1 with 35 ft radiator and 17 ft counterpoise wire that I have used with some success. Both antennas tune easily on some bands, but seem finicky on others. I have not yet experimented with different counterpoise wires due to the added complexity for backpack-portable operations.

Transceivers are a KX2 with internal tuner and a TX-500 with a ZM-2 tuner.

Might counterpoise wires of different lengths help with tuning on problem bands?

Well... yes see, when it comes to (not so) "random" endfed antennas, since you can't change the length the only thing you can do is working on the counterpoise, as for the length there's no "rule" although usually a good starting point for that is a length which is about 1/5 of the antenna length, also notice that on some bands the difficulty to find a match may be due to the CM currents causing problems to the ATU, now, if you are using a counterpoise, the easiest fix for CMC will be adding a decent choke to the feedline; also, the feedpoint height from ground will play a role.

Let me try modeling the 71ft in NEC; I'll assume that the feedpoint is at 2m from ground and that the antenna is sloping up with the far end at 6m from ground (if the setup is different I may model it, just holler ) ... anyhow, with such parameter, running a quick NEC model of such an antenna and running the optimizer to get a good counterpoise length value shows that a good counterpoise may be 11m (36ft) long, the config I tested is the following one

rand01.png (216.83 KiB) Viewed 6365 times

the "violet" dot is the feedpoint, as you see the antenna is sloping upwards as described above, while the counterpoise drops down on ground and then is laid out on ground, with such a setup the simulated SWR curve shows the following

rand02.png (85.72 KiB) Viewed 6365 times

and is quite promising, the highest SWR (above 10:1) is around 6MHz but, with that counterpoise, the antenna shows acceptable SWR on the bands from 80m to 40m, then the SWR curve lowers and the tuner should be able to easily find a match, again it's just a simulation, so all it does is giving us a starting point, but then some experimentation/testing will be needed, so my humble suggestion is to test that counterpoise at/near home and then play a bit with the length until you'll be satisfied, again it isn't "exact science" also since we aren't dealing with a resonant antenna, what we need is a low impedance path to allow return current to flow

As for the choke, while not mandatory for outdoors/portable operation, and if one uses a counterpoise, it won't hurt, it can be wound using the coax you have (or a piece of RG-316), what you'll need is an FT114-43 (for QRP it will be ok, willing to use more power use a 140 or even a 240) on which you'll wind 13 turns (6+crossover+6) as shown below



since we're dealing with portable operations, the coax won't pick up so much noise, so the choke may be placed at the RTX end of the coax

HTH