Open wire on cement / soil / metal - W6NBC was right

Status
Not open for further replies.

nanZor

Active Member
Joined
May 28, 2009
Messages
2,807
I think I agree wholeheartedly with John, W6NBC about the "fear" of using open-wire line near ground or even close to metal.

I ran across his testing well after I did some of my own (with twinlead, some old RS 300 ohm stuff, and a leftover chunk of 450 ohm window line). I was playing with zipcord as a feedline, and yes of course it DID have the inherent loss that has been recorded for years, but I was amazed that it didn't seem to be so touchy - even running around indoors on the floor!

I ran across his testing here:

http://w6nbc.com/articles/openwireline.pdf

I think I had seen this before and doubted it - largely because my own efforts at using open-wire were stymied by not having my own common-mode on the open-wire choked! Once I got common-mode under control, the open wire became usable - even laying very close to soil and concrete even.

The keyword to making this work is to cure your common-mode current in any way you can. For the longest time I thought that "balanced line" to a "balanced antenna" was a magical cure-all for common-mode. Not so - maybe because I could visualize how common mode works on coax, I couldn't visualize that a balanced 2-wire conductor can carry that common-mode current as well.

This could open up a wider world of possibilities for some antenna projects. Just take care of common-mode issues first and I think you'll find open-wire "tame" if not taken to absurdity / unsafe levels. I'm not going to run 1KW on twinlead on the carpet. :)
 

bharvey2

Member
Joined
Mar 12, 2014
Messages
1,842
I just watched a YT video about this very topic although the presenter was using a long wire with a 9:1 unun and a tuner. a Ground level antenna seems to do a fair job on the low bands (<10 Mhz), especially as it relates to reducing ambient noise. On the higher bands > 10Mhz) the antenna functioned quite poorly. He was testing on both receive and transmit working primarily the WSPR digital mode.
 

nanZor

Active Member
Joined
May 28, 2009
Messages
2,807
Well, here we're talking about using open-wire as a transmission line on the ground, as long as the common mode is kept at bay, and yeah, spaced away by some surrounding foam.

BUT, for the ground level antenna - I'd have to see that video. First problem is that transmit is a joke no matter what, unless you plan on having 30-50db loss be part of the plan. Ie, 1Kw transmit output resulting in qrp level EIRP. No thanks. Been there when the "grasswire" article came out many many moons ago.

In the other subforum here, I *have* used a 9:1 unun, even without a tuner for RX ONLY on the ground. Kind of a spin on KK5JY's dipole-on-the-ground, which I also have tested and can recommend for rx purposes if you are into that.

I took it to a different level with my own 9:1 ocfd on the ground version to test for the convenience of feeding at one end (for physical reasons - the antenna is still an ocfd) and compared it to a standard "center fed dog". Results were about the same.

Without going too far into it, since it is covered in that HF/MW monitoring thread, it is very simple. Instead of a DOG, or center-fed dipole on ground, using the 9:1 just offsets it, and works well if you need the so-called "end fed" physically. But there's a BIG catch.

No matter where you feed the DOG, be it center fed or offset with a 9:1, you don't want to exceed a half-wavelength at the highest frequency of use, or you'll go with screwy patterns, or just a total NVIS cloud burner at higher freqs. Which is also bad since there isn't any nvis at 14 mhz! :)

So for example, when I wanted 160 to 20 meters coverage (1.8 to 14 mhz), my total wire length was only 35 feet. Since I was using the 9:1 instead of center fed, the 9:1 unun was placed after about 20 feet of wire. Then a 15 foot coax jumper ran to a GREAT common mode choke - a CMC-0510-r (rx only version from myantennas). After that was any convenient coax run back to the shack. (which by the way also had a CMC) The unun used the braid between it and the choke for one side of the antenna, and of course the wire for the other side of the on-ground ocfd.

What made it successful was that the common-mode had to be hit *hard*, and not just a snap-on, or stupid ugly balun. Hence the myantennas rx-only CMC-0510-r choke with great specs. Especially needed for 160 and AM BCB.

Above 14 mhz, being longer than a half-wavelength, signals could be heard but the pattern was degrading rapidly above 17 meters. I guess the key word is one has to juggle overall antenna length to maintain a decent pattern, and not fool around with common-mode choking with half-hearted measures.
 
Last edited by a moderator:

nanZor

Active Member
Joined
May 28, 2009
Messages
2,807
Long story short - unless one is willing to get common-mode under SERIOUS control, then many on-ground antennas have patterns that are just in reality some random-wire on ground thing.

More transmission line testing at 100 watts here: I wish I had a kw to fool with, but so far, using brown #18 zip-cord, or more accurately lamp-cord, low-height dipoles are no problem with the cable running mostly on my cement and brick patio. Unreal. I'm aware of the inherent loss itself with zipcord, but my main interest was common-mode.

The trick to making it work was once again common mode control. At each end of my "zip cord" feedline, I used a 4-pack of MFJ snap-ons, the larger ones, and threaded the zip cord through two of them at each end in a binocular fashion with about 4 turns inside each. The run is on the cement/brick.

While I intend to put some air-conditioning foam around it, so far it hasn't acted all that terribly. SWR seems to be under control no matter how I snake it around. And of course durability is poor, so I'll have to pull out another run from my wire spool of it in a year or so.

I also have some major transmit-capable chokes at the rig itself too.

So that really seems to be key - I've even run it inside, but man if you don't choke it, it is just like unchoked coax.

Needless to say, "voltage baluns", are DEFINITELY out. Good ferrite and current-baluns are IN. :)
 

W5lz

Active Member
Joined
Feb 28, 2019
Messages
617
... another trick to remember is that twisting that ladder line/zip cord into a spiral will tend to even-out some of the unequal currents in that type feed line...
 
Status
Not open for further replies.
Top