I want to use an end-fed wire antenna, but what would be the best length of wire for the 20m and 40m bands?
Is there a way to work this out?
This is assuming a tuner will be used.
The length doesn't matter much. If you make it the right length, then it will present a good match to your feedline, but if you have a tuner, and either place it near the antenna or use a low-loss feedline, then that doesn't matter.
It's also possible to get really unlucky and pick a wire length that's outside of your tuner's range. What these lengths are depend on your tuner and also the wire's surroundings and also the ground system. Odds are that most lengths are fine though, so the easiest solution is to pick a length based on something else (like, the room you have available) and if you get unlucky, roll the dice again.
Also, if the wire is too short, then it won't be a very effective radiator. If the wire is longer than half a wavelength, then making it longer doesn't make it any more effective. Making it shorter than half a wavelength doesn't suddenly stop making it working either, so if you don't have enough space to put up a full half-wavelength that's fine, too.
Fact is, if you can effectively couple RF current into something that's large relative to the wavelength and is a good conductor, you get an antenna. The bigger problem with end-fed antennas is subtle: it's actually only half the antenna. The ground is the other half, and I don't mean Earth. I mean whatever is attached to the other half of the feedline. If you are using a tuner that has just one wire coming out of it, then the other half is the tuner's chassis, and whatever is connected to it. If you are concerned about making a good antenna, my advice would be to learn more about end-fed antennas work, and interact with ground, before worrying about length.
I have used an End-Fed Long-Wire antenna for many years. I have used it on 80M, 40M, 20M with 1:1 SWR... I did NOT try for the shortest long-wire possible. I did use a Long length which is several Odd Quarter-Wave-Lengths of my target bands 40M and 20M.
It is 186 feet long, up 17 ft, in the shape of a letter "Z"
and controlled by a Antenna Tuner and fed via 52 Ohm Coax, 32 ft long.
It is a copy of the BASIC SIMPLE LONG WIRE ANTENNA
found at w8ji website for antennas
My Long Wire has aprox. 11.5 quarter wave lengths on 20M. My Long Wire has aprox. 5.5 quarter wave lengths on 40M. Being aprox. odd quarter wave-lengths of the target band is important. The Antenna Tuner will load an End Fed Long Wire effectively, especially if it is cut to aprox. an Odd Quarter Wave-Length of your target band... 186 ft / 16.5 ft and 186 ft / 33 ft... The entire antenna is #12 stranded insulated copper.
The Antenna Tuner is a 1977 HeathKit HFT-9, a small QRP size tuner of the Cap-Coil-Cap double"L" design. Super simple. I use an MFJ-813, QRP-SWR-WattMeter calibrated model, and run 1W commonly.
As for effectiveness, I have worked most Europe, All States, and Hawaii at QRP/p 1 W with this antenna.
For Tuning to get more bands, as an experiment, I simply wrapped up 5 or 10 or 15 feet of the end in a tight loop, wrapped tape around it, and let it dangle. The Tuner brought in 80M, 40M, 30M, 20M, 21M, at SWR 1:2 and less. When adjusted slightly for 40M and 20M, the SWR can be 1:1 always, as the tuner is adjusted slightly to get the entire band.
The "other half of the Long-Wire" is called a "Counter-Poise" which can be aprox. 10% to 15% of the long-wire length. My Counter-Poise is aprox. 23 feet long, and dangles away from my Long-Wire Antenna. ... Counter-Poise is NOT grounded to anything. Nada, Zilch, No Ground. W8JI writes "adding ground rods can decrease RF efficiency when an insulated counterpoise is used." I have verified this by experiment. Inside the Shack, at the Antenna Tuner, there is a solid earth ground for the all the equipment, including the ground of the Antenna Tuner. ...
This is similar to the standard Antenna we use at all our Field Day Installations with great success. We toss our Long-Wires over Tree tops and dangle the counter-poise away from the Long-Wire.
... Read the easy text written by W8JI .
... Hope that helps. Glen Ellis, K4KKQ, CW/QRP 59 years.
I have used both 107 and 71 ft lengths, but didn't see much difference as my old KW At-230 tuner tuned both just fine.
I fed them through a 9:1 balun and 50 Ω coax with 25 ft of coax back under wire, possibly acting as counterpoise, but I'm not sure about that.
Everything is well grounded in my shack. It worked pretty good on 40 & 20m.
you may, in principle, use any wire length, foreseen you bring it to resonance with a suitable tuner. A simple and versatile QRP tuner circuit was proposed by WB3GCK (https://www.qsl.net/wb3gck/tuner.htm) (when rebuilding this circuit, I recommend letting the coil end open, instead of closing it to the variable tap, in order to avoid any shorted induction coil). This circuit works for capacitive and inductive load low impedance, as well as for high impedance wire antennas. It needs a counterpoise (including the mantle of the coax cable, the TX and the operator, hi). A 40m long wire is about multiple half wave (i.e. high-impedance) on 80m, 40m, 30m, 20m, 17m, 15m, 12m, and 10m A 20m long wire is high-impedance on 40m, 20m, 15m and 10m, and low-impedance on 80m, 30m, 17m, and 12m A 27m long wire is of medium impedance on all bands. A single strand of insulated outdoor telephone wire (comprising 3 copper and 3 steel wires in a double-layer insulation) makes a superb QRP antenna, because it can be tended free-hanging over trees and rocks. 73 de Edgar, HB9TRU
Yes, it is possible to even make very far contacts on random wire antennas.
See this site for information about the worlds simplest transmitter circuit,and how he was able to hear it from more than 10 miles away on less than 10 milliwatts with a wire outside his window going to trees.
Just put a variable capacitor (300pf or so) in series with the antenna and tune around for maximum power output without swamping the oscillator.
I enjoy connecting random wires to my bicycle mobile antenna, which consists of a 10 foot long PVC pipe.
For the best length, question I say use a 3/8th wave wire; a good compromise between the low impedence quarter wave, and high impedance half wave.
The OP's question may have been edited, but I didn't read the word 'random' in.
That said, with such a broad term as 'wire antenna', the 'best length of wire' depends on what you want to use it for. Some wire antennas with a vertical polarisation like a quarter wave vertical have to have lower take-off angles than a horizontally polarized like a dipole made from wire strung up between the trees in a back yard.
To calculate the lenghts: a rule of thumb for a dipole wire antenna length is:
dipole length (in feet) = 468 / f (MHz)
So that will be 32.96 feet for a dipole on 20m (each leg 16.48 feet) and 65.92 feet total length for a 40m dipole.
For a Loop antenna the length also depends on the shape of the loop (Triangle, Rectangular, Square, Round) but start with a full wavelength for the band you wish to work on and tune from there. With a loop antenna I also found that a loop resonant on 40m it will tune up on 80, 20, 15 and 10m but I recently changed to half the size (1 wave length on 20m) and it no longer tunes up on 15 and 10.
An excellent receive antenna for the lower bands is the Beverage antenna. If you have the space to put one (or more) up you will notice that this antenna is very directional and very low noise.
I have found wire antennas to be a great (and often low cost) way to experiment with antenna designs.
You mentioned you already have a tuner, so your antenna designs will not have to be 100% perfect, another reason to just have a go at it.
"...I want to use an end-fed wire antenna, but what would be the best length of wire for the 20m and 40m bands? Is there a way to work this out? This is assuming a tuner will be used..."
Using a tuner, the best length NOT to use is a quarter wavelength, or odd multiples (3/4, 5/4, etc.) The best length is a half wavelength, or even multiples (1/2, 1, 3/2 etc.). Of course, that may be hard to get the best wavelength on one band and not the worst length on another band. My advice is to use a wire that is between 1/2 and 3/4 wavelength on the lowest band (40 meters here). Say, 12 meters long. To Mike and SDSolar - the point is that quarter wavelength wires may exhibit just what you mentioned - an impedance that is out of the range of your tuner.