RF "in the shack" with Random Wire 9:1 Unun

[W6AIT]

Has anyone had "RF in the shack" issue with trying to operate digital modes using a Random Wire antenna? I'm using a QRPguys 9:1 unun with a counterpoise and 3ft of coax between radio and antenna, operating at 5watts. Using the cable included with my digirig I get errors on transmit, they say are related to RF in the shack. However, using my own USB cable eliminates the issue, which I find odd.

My searches last night had forum posts say that EFHW/LW & Random Wire are notorious for producing RF in the shack even with a counterpoise, has that been anyone else's experience?

[Andrew (grayhat)]

Has anyone had "RF in the shack" issue with trying to operate digital modes using a Random Wire antenna? I'm using a QRPguys 9:1 unun with a counterpoise and 3ft of coax between radio and antenna, operating at 5watts. Using the cable included with my digirig I get errors on transmit, they say are related to RF in the shack. However, using my own USB cable eliminates the issue, which I find odd.

My searches last night had forum posts say that EFHW/LW & Random Wire are notorious for producing RF in the shack even with a counterpoise, has that been anyone else's experience?

ooooh well... random wire

first step, go here

https://www.hamuniverse.com/randomwirea ... ngths.html

and pick one of the "good" lengths (green numbers list), that will at least ensure that your "random" won' t present a too high/low impedance on some bands; pick the length which fits your available space and considering that, to be efficient on the lowest band you want to cover, it will need to be above 1/4 wave on that band

next, pick a run of wire for the counterpoise, measure it to be at least 0.05 wave or more, the counterpoise is important, and is the only way, aside from raising the antenna, to adjust a "random"

now, with the above in place and with your 9:1 UNUN raised up as much as possible, and with your counterpoise going down to ground and then laying just on ground, you'll just need another bit

a pair of good CM chokes, they can be easily built using two #43 material toroids; FT140-43 will do ok for QRP, to wind them to form 18 turns, evenly spaced, of coax around the toroid, place a choke at the point where your coax, coming down from the antenna, reaches the ground, and a second one at the point where the coax enters your shack

if you'll still have issues, just holler

[Andrew (grayhat)]

an almost "classic" setup which I tried (several times) is the following

a 34ft antenna wire, connected to a 9:1 UnUn, the feedpoint (unun) of the antenna raised to 29ft, a counterpoise made of insulated wire dropping down (for 29ft) to ground, at that point the counterpoise wire is connected to two other 13ft wires running on the ground, perpendicularly to the antenna wire; a choke at the bottom of the feeding coax dropping from the UnUn and a second choke right before the coax enters the shack outside wall

such an antenna will cover (with a matching unit) all bands from 80 to 10 meters and will tune, with limited efficiency on 160 meters

HTH

[W6AIT]

ooooh well... random wire

first step, go here

https://www.hamuniverse.com/randomwirea ... ngths.html

and pick one of the "good" lengths (green numbers list), that will at least ensure that your "random" won' t present a too high/low impedance on some bands; pick the length which fits your available space and considering that, to be efficient on the lowest band you want to cover, it will need to be above 1/4 wave on that band

next, pick a run of wire for the counterpoise, measure it to be at least 0.05 wave or more, the counterpoise is important, and is the only way, aside from raising the antenna, to adjust a "random"

now, with the above in place and with your 9:1 UNUN raised up as much as possible, and with your counterpoise going down to ground and then laying just on ground, you'll just need another bit

a pair of good CM chokes, they can be easily built using two #43 material toroids; FT140-43 will do ok for QRP, to wind them to form 18 turns, evenly spaced, of coax around the toroid, place a choke at the point where your coax, coming down from the antenna, reaches the ground, and a second one at the point where the coax enters your shack

if you'll still have issues, just holler

Thanks for the feedback, that seems to be inline with what I've read. I did review that website and go with the 29ft antenna wire that's suggested there as well as on the QRPguys site for their 9:1 unun. I haven't had any issues tuning the antenna on 10-40m. I think you might have keyed in on something that could be big, my 9:1 unun is low to the ground. My shack is chest height below ground, so when I'm running coax out the window it's only 12-18in off ground. However, no part of the antenna, except the counterpoise touches the ground except for the coax touching the windowsill. With only 3ft of coax I'm going to need to invest in much longer pieces to install some chokes.

Thank you very much for your help, appreciate all the advice. Going to hopefully do an activation this weekend and see if I can avoid some of these issues with better antenna placement in the field prior to choking the coax.

[Andrew (grayhat)]

You're welcome; what I wrote is straight from experience; in your case, a quick fix would be using a good choke after the unun and making the counterpoise longer or adding more counterpoise wires, let me know how it goes

[Andrew (grayhat)]

ooooh well... random wire

first step, go here

https://www.hamuniverse.com/randomwirea ... ngths.html

and pick one of the "good" lengths (green numbers list), that will at least ensure that your "random" won' t present a too high/low impedance on some bands; pick the length which fits your available space and considering that, to be efficient on the lowest band you want to cover, it will need to be above 1/4 wave on that band

next, pick a run of wire for the counterpoise, measure it to be at least 0.05 wave or more, the counterpoise is important, and is the only way, aside from raising the antenna, to adjust a "random"

now, with the above in place and with your 9:1 UNUN raised up as much as possible, and with your counterpoise going down to ground and then laying just on ground, you'll just need another bit

a pair of good CM chokes, they can be easily built using two #43 material toroids; FT140-43 will do ok for QRP, to wind them to form 18 turns, evenly spaced, of coax around the toroid, place a choke at the point where your coax, coming down from the antenna, reaches the ground, and a second one at the point where the coax enters your shack

if you'll still have issues, just holler

Thanks for the feedback, that seems to be inline with what I've read. I did review that website and go with the 29ft antenna wire that's suggested there as well as on the QRPguys site for their 9:1 unun. I haven't had any issues tuning the antenna on 10-40m. I think you might have keyed in on something that could be big, my 9:1 unun is low to the ground. My shack is chest height below ground, so when I'm running coax out the window it's only 12-18in off ground. However, no part of the antenna, except the counterpoise touches the ground except for the coax touching the windowsill. With only 3ft of coax I'm going to need to invest in much longer pieces to install some chokes.

Thank you very much for your help, appreciate all the advice. Going to hopefully do an activation this weekend and see if I can avoid some of these issues with better antenna placement in the field prior to choking the coax.

the point is rather simple, the "random" is a compromise antenna, so one can't compare it to a "beam" or to a resonant antenna, but that doesn't mean that the "random" couldn't serve you well, and for that to happen, some attentions are needed; first, add some counterpoise, I found that a single insulated counterpoise wire, connected to the unun and going down to ground, joined at its end with two more wires to form an "inverted T" shape, works rather well, next ... a lot of pics show the "random" with a low feedpoint directly connected to a matching unit, the problem with that config is that, while for "on the run" QRP setups it will work, willing to use it at home would mean bringing the ANTENNA inside the shack, which isn't exactly a good idea, plus, the low feedpoint also means higher ground losses, so... if you don't have a remote ATU which you can raise up, the solution is using a 9:1 unun raised up as much as possible, both the counterpoise and the coax will then drop from the feedpoint to ground, at that point, the counterpoise will connect to the two wires forming the T, while the coax will connect to the first choke, this way. the vertical portion of coax will act as a counterpoise too, next the coax will run toward the shack entry. at that point it will connect to the second choke and, optionally, the coax braid may be grounded, and then a last run of coax will enter the shack

the above is just from direct experience, so it isn't "perfect" but, at least, should be a good starting point

[Andrew (grayhat)]

also, regarding chokes, here's a reference which may be useful

http://www.karinya.net/g3txq/chokes/

HTH

[N8TGQ]

If the problem goes away using a different USB cable, why not use that cable?

[Andrew (grayhat)]

Thanks for the feedback, that seems to be inline with what I've read. I did review that website and go with the 29ft antenna wire that's suggested there as well as on the QRPguys site for their 9:1 unun. I haven't had any issues tuning the antenna on 10-40m. I think you might have keyed in on something that could be big, my 9:1 unun is low to the ground. My shack is chest height below ground, so when I'm running coax out the window it's only 12-18in off ground. However, no part of the antenna, except the counterpoise touches the ground except for the coax touching the windowsill. With only 3ft of coax I'm going to need to invest in much longer pieces to install some chokes.

Thank you very much for your help, appreciate all the advice. Going to hopefully do an activation this weekend and see if I can avoid some of these issues with better antenna placement in the field prior to choking the coax.

Ok, had some free time and modeled it using 4NEC2, the results are in the image below (full size here)

rand29.png (100.08 KiB) Viewed 6547 times

the antenna has a radiator length of 29ft and is placed horizontally at 17ft from ground, a counterpoise drops from the feedpoint (9:1 UnUn) to ground and there is connected to two 1.37ft wires forming an inverted T (about 2.7ft overall) laying on ground, the coax drops down from the UnUn along the vertical wire and at its bottom there will be a 1:1 choke and the coax will then run toward the rig; the resulting antenna should have an SWR curve similar to the one visible to the right of the pic, which should be ok for most ATU units

By the way, that's just a simulation, so the real thing may/will vary

HTH

[edit]

here's the radiation pattern reported from the NEC simulation



in case you'll decide to test that antenna