RF from homebrew open wire feedline

[Tim Delaney]

Hi folks,

First of all, thank you very much for the add to the group. I've only been a licensed ham for a bit over a year and am just getting into HF. I wanted to start out with a hands-on experience I could handle, but I've run into a small snag. I put up an 80m skywire loop, which is about 25-35 feet off the ground, fed with homebrew ladder line. All wire is 12g stranded copper with plastic sheathing on the wire. I used electric fence insulators to make the spacers, about 3 1/2" spacing and the total length of the feed line is about 36 or 40 feet (I'll get a better measurement).

While the reception appears fine, when I attempt to transmit, it appears there is no propagation, just RF in the house: so much so that it takes the wireless Internet modem offline temporarily. That's why I'm guessing RFI. I've sought the advice of a couple other local hams: one said swap the open wire for coax and the other said lengthen the feedline to about 60 feet.

Does anyone in the group have experience with this issue, and if so, what did you do to rectify it? I'm using an MFJ 969 roller inductor tuner with its internal 4:1 balun. Left to my own devices, I'd lengthen the feed line first and if that didn't do the trick, I'd look at coax.

Thanks for any advice you may have to offer,

73

Tim
VA1TIM

[Andy]

My reply is not from experience, but just basic principles, so take it with a grain of salt.  There is no reason why the feedline would be radiating RF, if the signal on it is balanced.  That requires both having a balanced or floating load (which the loop should be as long as there's no accidental connection to ground, or close proximity to something like a metal roof), and if the balun at the feed end is balanced.  Swapping the feedline for coax might make it worse instead of better because the coax shield will radiate unless there is another balun at the far end where it connects to the loop, so of course you would need to add one there too.

The tuner's built-in voltage balun is OK but maybe not the greatest.  None of them is perfect.  You could also try adding a homemade choke "balun" (bal-bal?) between the tuner and feedline, to further block common-mode current at that end of the feedline.  (Wind a twisted pair around a toroid, or a ferrite rod.)

How do you know there is no propagation?

Is the reception really good, as in, clean signals with no noise pickup from local sources?

I think the kind of balun that's inside the MFJ tuner requires a good RF ground connection there.  I worry that this ground wire itself can radiate.  Personally I prefer situations where no ground connection is needed (except for safety).

Regards,

Andy

[Tim Delaney]

Thanks Andy, you're giving me more food for thought. I don't know for sure that I'm not propagating... I think I'd better get a nearby buddy to listen on the common frequency. I assumed as much because when I keyed long enough to call CQ, it took the wireless internet modem offline (and made XYL cranky!).  There is very little noise on receive: virtually none on 160, a bit of background noise on the other bands, but pretty quiet overall. Whether the feedline going through my workshop area into my "shack" is too close to metal objects or electricals may be another issue. It's several inches, but not feet.

This is one of those cases where a newbie needs a bit of success... so far I'm kind of hitting a wall... With help from folks like you, I'm sure I'll sort out the issue(s). 

Thank you once again for the advice, I'll fiddle around with a few more things. If anything else comes to mind, please don't hesitate!

73,

Tim

VA1TIM

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[Ronald Johnson]

Tim and Andy  ​... I operate 270 ft loop...a 3 ft jumper from xmiter to a MFJ-915 RF Isolatoranother 3 ft jumper to a MFJ-969 tuner. 35 ft of RG-58 out the window under the roof ofthe back porch to a MFJ-1601 laltterline end insolator, that has a SO-239 coaxial cpnnector on it. 40 feet of 450 ohm  ladderline connected to it at about 20 feet off the ground. 14 ga copper single line wire is used. swr is at 1 to 1.5 all bands.100 watts max. this is the best ant I have ever used. 

[Tim Delaney]

Hi Ronald and Andy, 

Once again, you guys have got me thinking. I've got a 3 ft. jumper going from xmitter to MFJ-969, then open wire from tuner out to the antenna. According to the tuner, I can get a flat SWR which is confirmed on the FTDX-1200's SWR meter. Unfortunately, I don't think I'm propagating a signal and it puts my internet modem offline. I just rebooted the modem a few minutes ago. Where would you start? Lengthen feedline to say 60 ft; replace the open wire with coax; get an MFJ-915 to try and take out RFI in the house? What's a logical first step (that's likely going to fix the problem and get me on the air)?

Many thanks!

Tim

On Mon, Apr 9, 2018 at 7:52 PM, Ronald Johnson <ronald.j...@gmail.com> wrote:

Tim and Andy  ​... I operate 270 ft loop...a 3 ft jumper from xmiter to a MFJ-915 RF Isolatoranother 3 ft jumper to a MFJ-969 tuner. 35 ft of RG-58 out the window under the roof ofthe back porch to a MFJ-1601 laltterline end insolator, that has a SO-239 coaxial cpnnector on it. 40 feet of 450 ohm  ladderline connected to it at about 20 feet off the ground. 14 ga copper single line wire is used. swr is at 1 to 1.5 all bands.100 watts max. this is the best ant I have ever used. 

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[Andy]

Tim, it might be that there's nothing wrong with your ham radio setup.  If you are receiving well, then your signal is probably getting out too, maybe through the desired route (the loop antenna).  The modem is near the antenna, and it may just be that it, and the wires connected to it, are picking up too much RF.

Consider things you can do to make the internet modem more RF-immune.  Every wire or cable into the box is an ingress path for RF.  If you "choke each of them off" (by wrapping some turns around a toroid), it might keep the RF from getting inside the box.  I guess you could try clamp-on ferrites around each wire, but a simple clamp-on ferrite might not be effective enough on a low frequency like 80 meters, unless you can wrap the wire through the ferrite several times.  The more turns, the more inductance and the more effective at choking the RF.  Each cable into the modem box is half a dipole.  Start with the ones that are easy to do.

You don't need to split the wires apart before they go into the modem.  You are trying to choke off the common-mode signal, on each of those bundles, from getting into the box.  If there is an Ethernet plug, that is one thing to add a choke to.  DC power is another.  And so on.

Regards,

Andy

[Ronald Johnson]

Tim if u are running ladderline from your MFJ-969 in the house out to the ant, that should be the problem. The MFJ-915 should take care of the RF in the house. Feeding the loop with ladderline is great for the loop but not from inside the house "sometimes...Ron hope all that helped...

[Tim Delaney]

Ron, I'm quickly coming to that conclusion. I wish the shack was near an outside wall, but that isn't the case. I moved the line further away from metal and other electricals, but no change. I think the MFJ-915 is calling me!

Thank you all again for your help. I'll provide an u0pdate, once I get things under control!

73!

Tim

VA1TIM

[Ronald Johnson]

Hey Tim well I know that it cant heartand using coax to feed the latterline outside the house cleared it up for me...all u can do it try...let me know if it helps...73's Ron KC4EQR...

[Tim Delaney]

I'm ordering some coax and a balun today! I'll let you know how things work out.

Thank you once again,

Tim

[Ronald Johnson]

Tks Tim I would like to know how is works out...Ron

[Tim Delaney]

Hi guys,

Quick update: my shack in in the basement of our 2 storey home. My wife has an office on the second floor and the loop antenna gets as close as about 10' from the wireless modem. Yesterday morning, I thought RF from the feedline in the basement would probably have less of an effect on the modem than from the antenna itself! I moved the modem onto the main floor and tried the radio on 80, 40 and 20 m. The modem did not kick out. I heard a CQ from Barcelona on 40 m and was able to make a 59 contact with him. When I checked my power (bloody Yaesu menu systems!), I had the TX power set at 20 watts. Later that afternoon, I was able to join a regional net on 80 m and got a good report. I think it's a good idea to switch to the coax and balun inside the house, but it was nice to have some success.

I think it's interesting that there would be enough radiation at a sensitive frequency that would take the modem off line, and I wonder what, if any, interaction there was between the antenna and the modem, relative to transmission from the radio. It doesn't make sense to me, but things don't always make sense to newbies!

I'll keep poking at it until I figure things are stable and will keep you in the loop. There's quite an art and science to this hobby!

Tim

[Ronald Johnson]

well goodTim...Tks...Ron