With apologies to J.R.R. Tolkien and the Lord of the Rings books...
Probably the most common questions that came up when I was teaching ham radio
beginner's classes were antenna questions, and of those, the most common was
(paraphrasing), "why should I need more than one antenna?" My impression
is that people are used to carrying a portable radio with them, or even their
car radios, and they never had to even think about antennas let alone their physical size.
The answers are talked about in a piece two years ago on
Antennas, Tuners and Analyzers. As I said in that article, as a general rule, transmitters are
fussier about the exact impedance (electrical characteristics) of the antenna
than receivers are and you'll find that your transmitter might not put out
power at all - especially if it's a solid state amplifier (transistor of some kind). Some transmitters not only don't put out power if the antenna
isn't close to 50 ohms, they'll put out smoke instead of power and
never transmit again. Still, while you might be able to put up an
unwieldy bunch of wires or aluminum tubing at your house (and might not...)
there's a large group of hams who want one antenna system that does
everything. If you're not active in the "thousand hobbies with one
name," you might not be aware that there's a big interest in operating
portable, hiking in parks or up to mountain tops. These are called
Parks On The Air (POTA)
and
Summits On The Air (SOTA).
If you're backpacking up a mountain with a battery, a transceiver, a computer (Laptop, Tablet, Raspberry Pi, whatever) and pounds of other stuff, it's more difficult to take multiple antennas. Likewise, putting up a few antennas and positioning them as optimally as can be just ain't happening. Finally, you'd like something that doesn't need an antenna tuner - more weight and more to carry. It's all compromise.
Let me repeat my Three Laws of Antennas here.
First Law: anything you can put up works better than nothing at something.
Second Law: nothing is best at everything.
Third Law: whatever you can put up won't be as good as you'd like. Unless you have a Jeff Bezos-level budget including the property to match.
An antenna design that goes back decades but which hasn't been widely used since then is the End Fed Half Wave antenna, or EFHW. The EFHW is pretty much the "new hotness" in antennas. As the name says, it's a half wave long, so 468/frequency for the result in feet. The most popular version of the EFHW is cut for the 80m band, about 133 feet long, because they have the interesting property of presenting a good impedance for every HF ham band - after one critically important trick. I don't have an EFHW myself, but here's a plot of VSWR from a club in Greece that I found with an image search.
Notice that the markers are in every ham band from 3.5 to 28 MHz and the worst VSWR marker is 2.60 at 10.13MHz in the 30m band. Most radios can tolerate a mismatch like that, although many will reduce their output power. You should find that information in the user manual.
The critically important trick is that while the impedance of a center fed,
half wave dipole is close to 73 ohms, the impedance of a half wave when fed at
the end is much higher, thousands of ohms. What users do is add an
impedance transformer between the feedline (coax) end and the antenna
wire. A very common step up is 7 turns to 1, which gives an impedance
transformation of 72 or 49:1. That steps 50 ohms up to 2,450.
As law #2 states, nothing is best at everything, nothing is perfect after all,
and the trade with the EFHW is that the antenna pattern, the directions in
which it receives and transmits best, changes with frequency. Antenna
patterns are also strongly affected by the way the antenna is strung between
supports and especially the antenna's height with respect the
wavelength. In use, these tend to be thrown over tree branches and run
in almost random directions. As rough rule of thumb, the higher in
frequency the antenna is used, the more the strongest patterns approach the
direction the wire is pointing.
As always, you can build an EFHW yourself or buy one ready to string up.
Update 29 Aug 2022 0915 EDT: I neglected to add part 1 in the original title. Over another couple of posts, I plan to add some coverage of more types of multiband antenna systems.
Back in the '90s, friends in military service told me that "MREs" or "Meals Ready to Eat" were three lies in one. They weren't meals, they weren't ready, and you really shouldn't eat one. That's what I think of when I hear about "End Fed Half Wave Antennas," three lies in one. They aren't End Fed. You can't feed an antenna at the end. You must have something to source current from, and it isn't thin air. There is always a counterpoise, whether a wire or you and your transmitter. They aren't a half wave, except at the lowest frequency (and that only approximately). And it isn't an antenna, so much as a dummy load on many of the bands. Those 49:1 transformers put most of the power into heat on many of the bands. No wonder you never see efficiency tests on EFHW antennas.
ReplyDeleteThat said, an old law says "RF's gotta go somewhere." In fact many users make fine contacts with them. Their light weight, small size, and easy to use (no tuner required, sometimes at least) makes them very useful for the POTA / SOTA crowd.
It's a nasty trade, but electromagnetics is like that. Say you put 10W into the system and radiate 1W. Radiating 1/10 of your power is terrible. Pragmatically, though, you're radiating more power than if you didn't have an antenna at all. It's "First Law;" anything you can put up is better than nothing.
DeleteAnother aspect is that since P=I^2*Z as Z goes up, current goes down. Which means voltage goes up because P=I*V. At some power level, you're going to create coronas and then start arcing over.
I have a lightweight simple dipole made with surplus army telephone wire. I cut it for 40m. In testing with an analyzer I could tune it for 30m and 20m by just rolling each end up on kite winders. I marked the sweet spot for each band with tape. I need to test it more though, been rather inactive this past year.
ReplyDeleteSince dipoles tend to work on their 3rd harmonic, it probably also works on 15m right now - when it's set to 40m. Since every band is the third harmonic of something , your adjustable 40/30/20 dipole might get you on on even more bands. Different color codes to tell you where to roll up the ends to? It all depends on how much work you want to do!
DeleteI have a Big Stick antenna at 30 feet here in Austin. I can shoot skip all over the world with the 250-watt Linear amp mounted to my radio, although my neighbors can't watch TV when I broadcast, so I only talk later at night, lol
ReplyDelete