You’ve seen this article 8 times before – why the dipole is great, economical, etc. etc. – I think there’s more to it than that, and I’m going to put my own spin on it..
The wire dipole for HF is a thing of beauty. Not only is it easy and cheap, it really does work better than anything anywhere near the cost, plus – it’s fun, and it’s something you can really touch and feel (or get tangled up in, especially if it’s copperclad.)
I am not going to fill your head with dB measurements, height above average terrain, or (heaven forbid) the endless debate of “balun or no balun?” I am here to tell you that I’ve used them, over and over, for almost 40 years – to make contacts all over the world, using 100 watts or less. If I had to guess, I probably have way over 200 countries on dipoles alone. The rest were on verticals, usually on 80 or 160.
Here are my 5 rules for HF dipoles:
- never pay money for a “complete” dipole
- stay away from the G5RV
- use balanced feed only for educational purposes
- the center needs to be up as at least as high as the length
- feedline needs to be perpendicular to the antenna
1.) never pay money for a “complete” dipole
You only need three things to make a dipole – some wire, a center insulator, and 468/frequency. (Some feedline will help.) Example: 468 divided by 7.1 MHz = 66 feet. This is a good one to remember, because if you are like me, it’s easier to remember that a 20-meter dipole is half as long as a 40, and an 80-meter dipole is twice as long, etc. (Easier than doing the 468/f in your head every time.)
Even paying full-price for the best antenna wire money can buy (Wireman #531,) and a set of Budwig center and end insulators, that 40-meter dipole will cost you about $45. You can get by spending less than $20, if you don’t mind #18 copperclad. Most of us have a few of these things lying around, and end insulators can be made out of almost anything. (Old toothbrushes? PVC?) Just don’t try to use steel or aluminum wire!
Also, using “electrical” (THHN or similar) solid or stranded copper wire is OK, but only for shorter (20 meters and above) lengths. At longer lengths it will stretch, and probably break within a year or two. Copperclad is a fraction of the price, and does the same job.
2.) stay away from the G5RV
The G5RV, built as it was originally designed, was an antenna for 20 meters. The design was based on choosing odd (non-resonant) lengths of antenna and feed line, so as to cause the “least terrible mismatch across the band.” (My words.) All the rules still apply:
It isn’t going to work on a band for which it is less than a half-wave long. (A G5RV is 102 feet long.) That means for example, it might be OK on 40, if it is up high enough (see below.) It will never cover both ends of any band.
Do yourself a favor – Every time you see a “one-size-fits-all” antenna solution, walk away. Probably the best investment you could ever make is a multiple-position antenna switch, so that you can compare as many antennas as possible.
In all my years of using dipoles – especially balanced-feed dipoles (and loops,) I found that where it was possible (thanks to a tuner) to use the same antenna on multiple bands, there was still a trade-off. The dipoles still radiated best on bands they would be close to a half-wave for. Note the word “radiated” – this is what gets overlooked in all the advertisements. How well does it radiate? What is the actual efficiency?!
A properly constructed dipole is 90% efficient. Take a look at Doo-Dad antenna company’s product specs… (you know, the ones where it is “only 31′ long, and still has a great signal on 40 meters.”) Do you see “radiation resistance?” Nope, because it’s low. When you start “shortening” antennas, your are on your way to driving a dummy load instead of an antenna. It has a great SWR, so it must be radiating well, right? Nope.
In the end, the only thing that counts is, “what is the field strength?” This is a measure of actual power received from a transmitting antenna, in a certain direction, at some distance from the transmitter. It is not a manufacturing spec, and can only be determined from tests. Some of the more reputable manufacturers have actually done these tests and include the results.
It is also why modern network-based tools, such as the reverse beacon network, are superior to any test an individual can do in their yard. http://www.reversebeacon.net/
In my humble opinion, the best multi-band wire antenna is the fan dipole. It’s basically two to five dipoles, all connected to the same feedpoint. They can be upper and lower portions of a single band, or multiple bands. They work great, and there is no mismatch, or efficiency penalty. They are just a bit harder to support in the air.
3.) use balanced feed only for educational purposes
Don’t get me wrong – balanced feed is great. It is far less lossy than any coax, and it is absolutely your best bet, if you must try to make a single antenna work on more than one band.
I built, from scratch, a succession of balanced tuners because, when I started, the only commercially-available one was the Johnson Matchbox, and even those were generally not in great shape. Long live the doublet!
Except that, for everyday use there is a price to pay in convenience – not to mention the eternal puzzle of how to get the feed line through a concrete-block wall… One reason I outgrew them was that DXing requires being able to get on the air, and “tuned up,” FAST. A few extra seconds twisting those knobs can mean the difference between catching him before the others hear him, and being drowned out in the pileup. Did I mention I have never used more than 100 watts?
Finding those tuning solutions is endlessly educational. A lot of times, when I noticed that a particular antenna tuned really well with only a minimum of L and C, I would just grab some solid wire, and my collection of doorknob capacitors. Then I would wind a pair of coils over, let’s see, a can of V8. Then just experiment with taps on the coils, until it got down to 1:1. Then I had a quick, efficient (but no longer multiband) antenna.
4.) the center needs to be up as at least as high as the length
Go ahead, run those predictions of elevation angle versus antenna height. I can save you the trouble – the answer is, unless you are just trying to work stations on the same continent, you need your dipole up high. To be exact, the feedpoint/center needs to be up as high as possible, which is one reason why the inverted-V is so popular. The ends can slope down, or even drop straight down, and it will hardly affect the output. Most of the radiation comes from near the feedpoint. When it is less than 1/2 wave high, you are just heating up the dirt. (The same goes for verticals with not enough radials, but that’s another story…)
There is a special case, where at least the center of the dipole does not need to be up high, and it will still work really well: a vertical dipole. Most people don’t bother with this, because it is still difficult to hold one end high, and keep the feedline perpendicular. However, this method has seen great suceess, and is a favorite of DXpeditions.
A sloping dipole is essentially a vertical dipole, favoring one direction at a higher angle. What you might want is two or three slopers, aiming different directions, and of course, an antenna switch.
5.) feedline needs to be perpendicular to the antenna
This one often gets lost in the shuffle. It usually happens when one end of the antenna is attached to the building the shack is located in. This often results in something like a “sideways-Y.” This causes the currents in the antenna and feedline to be out of balance, which besides disturbing your radiation pattern, can result in RF in the shack. Maybe what you really want is a sloper, or a ground plane.
The carolina Windom, while not a dipole by any stretch of the imagination, can also work well in some space-constrained situations. It still needs two relatively high supports, but since it is fed from the ground, it can occupy a narrow strip. It will cover more than one band, and in most cases will beat a G5RV for DX.
I recommend dipoles for 40 through 10 meters – although my favorite antenna for 10 is actually a ground plane. (Make sure you get the radial kit.) It’s the most antenna in the least space on 10.