ottavio said:
Speculative question, because I haven't invested on the gear yet and, quite frankly, I don't know what I am talking about.
Assume I'm the wild (say, a park) and I have a cheap QRP transceiver that I can sacrifice (e.g. a Pixie or similar). Assume I choose the 30m band (10 Mhz). How do I go about and make it roughly resonant at that frequency and the SWR is not too high to kill the TX stage? How do I calculate size, diameter and number of turns to make a half-decent un:un?
Please spoon fed me because I find this very intimidating.
Several questions there...
To cover the title of your topic, without any test equipment it is nigh on impossible to accurately tune/match an antenna. With some antennas such as say a magnetic loop antenna,
once the coupling loop has been adjusted accurately it is possible to tune the antenna by ear. You could simply listen for the maximum background noise while adjusting the tuning capacitor for the loop.
It is possible to make dipole antennas that would be 'in the ball park' of a desired frequency simply by using a calculator, but ideally they would still need to be fine tuned to get the best match/efficiency from them.
For most simple antennas (dipoles, half wave, quarter wave), simply divide 300 by the frequency in megahertz and that will give you the wave length (a full wave). Next you need to remove 5% (approx) to allow for what is called the velocity factor of the wire, which just for fun varies depending upon the length to diameter ratio of the wire used. The 'accepted' correction factor is 5%.
After that just divide it by whatever type of antenna you want. Dividing it by two will give you the approximate length for a half wave, which you could either end feed, or else split it in to two and centre feed it (a half wave dipole).
If you divide the result of the initial calculation by four that will give you the approximate length for a quarter wave antenna, which you could either feed against ground (quarter wave of wire plus ground stake), or else you could have a counterpoise system to feed it against (quarter wave of wire plus some wire radials).
Half wave dipoles are generally easier to to tune/feed/load than some of the other antennas mentioned because they are able to operate independently of the ground. As soon as you start to use counterpoise systems, end feeding half wave antennas and the like it can get very messy very quickly due to the vast number of unknown variables involved.
If nothing else, i.e. if you are prepared to make resonant antennas rather than relying on matching circuits to feed random wires, you really do need some way to measure SWR/impedance.
In the old days antennas were tuned for maximum smoke, or at least maximum antenna current. The simplest way was to put a small bulb in series with the antenna and adjust your transmitter to give the maximum brightness!
The simplest 'useful' device you could make is a "resistive antenna bridge". These can be very simply made, and are self-calibrating in a sense, if reasonable quality components are used. The only item likely to cost a few pounds (unless you have a well stocked junk box) is a sensitive meter movement. Something like 50uA FSD is ideal, but you could maybe go as high as 100-200uA FSD. Much more than that and it will lack the sensitivity required for QRP working.
A very cheap way to make quite a nice resistive bridge is to buy or obtain a CB Radio SWR meter. CB SWR meters are OK around 27MHz, but the design of the sensing line in them precludes their use much outside of their design frequency range (say 26-30MHZ). Above this range and they tend to become inacurate, below that range and they lack the sensitivity required due to the short length of the sense element.
But, if you obtain one, it makes a great source of components to make a resistive bridge! For a start you have the case/connectors, you will also have quite a good meter movement, and the calibration (sensitivity) potentiometer. With a few resistors and diodes etc. you can build quite a useful instrument. In use you just adjust the antenna/tuner for the
minimum reading on the meter.
All they do is split the RF two ways, one portion goes to a known load resistor (50 Ohms ideally, but you can also use 47 Ohm resistors, or 51 Ohm resistors without too much danger), and the other portion goes to the antenna. The bridge indicates the voltage difference across the two loads, hence when they are both equal you get no voltage difference, and so the minimum reading on the meter.
Just search the web for "resistive swr bridge" or see the attached PDF below (which is from the G-QRP website).
I actually built the bridge in their design, using an old CB SWR meter, and it worked fine. I still have it in the attic, and I do use it from time to time when using various magloop antennas (as it's so quick and easy to use).
As regards matching random wires etc. that is a whole nest of wasps! If you only want one band, use a resonant antenna such as a dipole or vertical quarter wave with a counterpoise system. If you want to play with end fed antennas I would suggest just simply buying an antenna such as the "EndFedz" ones, as they generally give a good match straight out of the packet and are quite forgiving of mounting.
You would be best to read up on dipoles, bazooka antennas, Zepp or Zeppelin antennas, and another useful one is the Fritzel antenna
https://pa0fri.home.xs4all.nl/Ant/FD4/fd4eng.htm
All of the above work well as single band antennas, the Fritzel will work on harmonic frequencies too.
Finally for now, you mentioned Un-Uns. They really do not help at all, especially if you are running QRP.
A 49:1 UnUn is generally used for end feeding a half wave antenna, but there is a lot more to getting it to work than simply winding one coil. There are a lot of other factors that do their best to mess up the tuning! If you want to read some useful info on an end fed multiband antenna, have a look at the Olandesina (little Dutch girl) design by IK0IXI
http://www.philipstorr.id.au/radio/seven/Olandesina%20story.pdf
I have built two Olandesina antennas over the years and found them to work well after trimming, but maybe not 'stable' enough for portable use (the info on the matching transformer and choke is useful though, in the above PDF).
Random wire ATU designs are ten-a-penny around the web, it's not worth going in to great detail about them here.
If you look around eBay for "qrp tuner kit" there are some neat little kits from China for around ?10 or so. These have the matching unit and a resistive bridge(?) SWR indicator built in too. They use an LED rather than a meter movement to indicate the match status. I'm sure these kits can be obtained from UK sellers too.
I think the design is floating around the web somewhere, as these units do not come with any printed manuals or assembly instructions, my thought was you could build your own 'clone' possibly.
Hope the above helps. bd
73, Mark...
