Questions about counterpoise for random wire antenna
I am working on getting an HF station setup. Will be operating outdoors to start with. I have a MFJ-949 tuner on the way, and I plan to use a random wire antenna.
I have read that I need a counterpoise. I've read that this can be a wire laying on the ground. And/or it can be a rod driven into the ground?
What also puzzles me is that the manual for the tuner says in the section about random wire antennas: "Do NOT ground the random wire antenna". Are they talking about the transmitting wire itself? If so - of course you wouldn't ground that!
Any other tips about a random wire antenna? I've read what lengths to avoid, I will probably use a 35 foot wire. Will be running 50 or possibly 100 watts. Maybe even 10 if I don't get a power supply worked out before my vacation trip.
-Forget the myths about special random wire lengths. They are bogus. A random wire is, well, random. Cut it for whatever length is convenient for you. ( you figured out the bit about not grounding the antenna).
- You can work it against ground. That's what the first hams did. Put a rod in the ground and run a wire from your tuner ground directly to the rod by the shortest length.
-If you want to improve performance add one or more counterpoises under or near the antenna attached to the ground rod. A fan shape under the antenna and slightly longer would probably be ideal, but again, do what is convenient.
In decades past this was a common antenna, particularly for newcomers and most hams understood how they worked, and learned how much better most other antennas are. Apparently now myths are developing about it.
The most common complaint when set up inside is having RF in the shack. Getting 'bit' on the mic, or when touching a cabinet, or having meters malfunction, etc. Keeping the feeds to the antenna and ground short and using counterpoises help with that.
Give it a whirl, you will have a connection back to times past. bill
Actually they're not. There are certain lengths which cause high impedances or reactance out of the range of tuners so need to be avoided. However other than that the rule of "longer is better" applies.
Originally Posted by KB4QAA
Look up the W3EDP - its what I use, from 40m-15m and I have a simple MFJ 901B tuner. I don't use a lot of power - 50w but thats enough. Its not the equivalent of a same hieght dipole or a beam costing a few hundred pounds, but it gets you on the air making contacts and having fun. I use it because its a VERY convenient installation at my house - the antenna runs out the back of the tuner, onto the slope of the roof, up to the apex, along the flat roof and then the last 30 feet or so runs to a pole at the corner boundary of our land, about 18 feet above ground level. The 17ft counterpoise runs from the earth on the tuner, out the window and into and along the gutter.
Sure I am looking at other antennas but for the moment its cheap, keeps Jill happy, and works.
If you use a long wire tuner, like the MFJ-16010, then the idea becomes to have a high feedpoint impedance. A halfwave is desirable, actually. This is because, at the feedpoint, you have little current--and thus you should have low current coming from ground. A short counterpoise is high impedance--but low enough to effectively couple the RF out of the tuner. A longer counterpoise won't hurt (and should help, really--simply a lower RF resistance to GND).
In the end, KCL still applies--if there is current being pushed onto the radiator, then there must be current pulled from some place (aka the counterpoise). It's just that, on a long wire that is some even multiple of a quarterwave, those currents are low.
I recall using a 100' wire on 40m and up (and 80m with very little performance). If you can measure the wire, pick on that avoids odd multiples of the frequencies you want. OTOH, I suspect your tuner won't like a full half-wave, so cutting to, well, absolutely random lengths might be best. But I'd run then with a quarterwave counterpoise for each band, if not a half-way decent RF ground.
Wire is just fine on the ground. A ground rod driven into the ground is ok for 60Hz, a start for lightning dissapation, and not very good for RF.
Let me clarify a point. A random wire antenna is one that is one that is cut to suit some criteria other than electrical. If you are cutting a wire to be resonant at some frequency then it ISN'T a random wire!
It may be a horizontal or vertical End Fed Wire worked against ground, or against a counterpoise.
And a Random Wire antenna doesn't mean the installation is sloppy or not well engineered.
At any feedpoint, there has to be as much current flowing out onto the antenna as flows back into the ground. Net currents flowing each way have to be equal.
If you use a poor ground, the balancing currents will flow though you radios, wiring, and everything else. That causes problems.
Originally Posted by KB0TKZ
Thanks for all the responses. I have a related question. I don't understand why there are 3 terminals on the back of the tuner for ladder line/wire ant./ground instead of just two.
What is the difference between the leads for ladder line and the leads for a random/long wire and ground? I don't understand why there isn't just two terminals, and you could either use those to hook up ladder line or hook up a ground and a random wire.
What if I wanted to locate the feed point of my random wire antenna outside the shack? Then would I use ladder line, and would I use the ladder line terminals or the wire ant./ground terminals?
It's nice to ground all the equipment together, and preferably to a good RF ground. So as to prevent getting zapped.
If the antenna is fully balanced, then yes, you don't need the ground lug on the tuner. But if there is anything to cause an imbalance, you can have a common mode current, in which case the tuner case can wind up with some RF on it.
As for relocating the feedpoint: this can be dicey. If the ladder line is an appreciable length, it will transform the impedance. But more importantly, if it's high impedance, then you are essentially adding shunt capacitance across the feedpoint, with short lengths of transmission line. Not a good thing. if the ladder line is sized for a quarter wave length, it can be used to feed a halfwave radiator, though, as it will transform a high impedance down to a more tuner-friendly impedance.