This article is condensed from a trio of postings that I made to USENET some time ago, and contains some extra followup information to those articles. It describes the construction of a simple dipole antenna suitable for use with a handheld scanner such as the AOR AR8000, and describes some of the benefits of using such an antenna.
This struck a chord, because I knew I had all sorts of electronic odds-and-sods at home, including a soldering iron and a number of defunct 50ohm Ethernet terminators which could be used to connect direct to the AR8000's socket.
I left without buying a "replacement" antenna for the AR8k, reasoning that I could come and get one later, and decided instead to experiment with homebrew antennas for a while.
Actually, in the end I replaced the stock antenna with another whip, as well as building two other antennas and buying yet another whip, bringing my complement up to 5:
The W881 seems better at some signals than the W801; I tried a few empirical experiments, with signals in the 6m (50MHz) and 2m bands, as well as images of BBC FM signals down around 70..88MHz; the W881 would typically add a couple of bars to the signal strength meter histogram in most cases, and with the images it was only possible to pick up some of them using the W881 instead of the W801.
The "super gainer" really does seem to live up to its name; with moderate to strong signals it seems to manifest the boost as extra clarity of tone in the spoken voice, but the hi-fi buff in me is so appalled by the AR8K'S perpetual background hiss that it's not something I'd crow about.
Actually, this is my one major gripe about the AR8000 after the antrnna is disposed of; I tend to listen through a mono adapter and a pair of ancient but comfy Sony MDR-V7's that have a very pure/wide range, so the hiss bothers me a lot until the neural filters kick in after a few minutes.
One thing I do worry about is that the W881's extra length may make it potentially more vulnerable to damage, eg: I wouldn't just toss it into a shoulder bag when traveling, so I tend to use it only in places there I don't have to worry about mangling it, else I use the W801.
The W881 also may do better than the W801 towards the HF end of the spectrum, but I actually use something different for that, a...
This was my first experimental antenna: I just took some bell wire, split it into a single core, and soldered one end to the pin of an old Ethernet terminator that I had hacksawed open and removed the load resistor.
Then I used hot-melt glue to add some structural integrity and threaded the wire through a hole drilled in the plastic cap of the terminator to make a nice solid unit.
This works very well, and IMHO is the only decent antenna out of the lot of them for MW listening, allowing me to copy weak stations all around Europe. It gives semi-adequate SSB mid-atlantic airband reception too; this may enthuse me into saving up for an AR7030 someday.
You can either plug this antenna straight into the scanner or hang it diagonally across the ceiling and use a short length of Ethernet cable as a feeder (so you can sit comfortably) - though I shall soon make a proper feeder once I work out what cable to best use.
Some people say that using coaxial (Ethernet) cable as a feeder for an unbalanced random long-wire antenna is wrong, but I gather that the cost of a balun to make the system balanced is prohibitive and I am having problems tracking down a supplier regardless.
Moreover, I'm not certain how much improvement using a balun would bring to this rig, given that my "feeder" is only 6 feet long, and wonder whether any improvement would actually justify the cost - or if this is just a "purist" thing.
The magical ingredient in this was a small strip of "choc-block" - those small electrical connectors that look like "LEGO" and have a screw on each side of a contact block.
I took a 2x2 piece of this and a straightened out coat-hanger, which I arbitrarily cut into two 44cm lengths, and then sandpapered the enamel off one end of each length and bent over about 7mm of bare metal into a right angle.
I then soldered a couple of short drop leads onto a male BNC connecter (the leads are no more than a couple of inches long, probably the shorter the better) and mounted everything into a cheap plastic project box about the size of a large matchbox. Then I screwed the leads into the chock block, connecting the dipole arms into the other side:
...and used an old cork, cut in half and shoved on the end of each wire, to stop them bodging holes in things like curtains, walls, kids, eyeballs, etc.
Lacking expensive coax for such an experiment, again I joined it to my AR8000 using (again) six feet of spare Ethernet cable; this is where you could economise even further by baring the shield and core of the coax and screwing it directly into the choc-block; I am not in a position to mount one of these things permanently in my house, so it is useful for me to be able to remove the feeder and store it separately from the rest of the antenna - hence my use of an male BNC connector (cheap, bought from Maplins) in the design.
The results are interesting: it's very effective when pointing in the right direction, with reception/gain much better than my W801 and even somewhat better than the W881; eventially I got fed up with waving it around and mounted it vertically on the back of a door in the spare bedroom/shack, and have barely moved it since.
The squelch has to be set slightly higher when using the dipole, but I don't know if that's due to the overall gain of the system picking up more noise, or if it's because it's an unbalanced dipole.
In it's first incarnation, the dipole was made up of two 44cm lengths of wire cut from a single coathanger; this was simply because I didn't know if it would work, and couldn't be bothered to cut up more than a single coathanger, and 88cm was the maximum length I could get out of one.
My reference book (Antennas for VHF and UHF, I D Poole, pub Babani) helped me calculate that this first dipole was resonant around 170MHz; this surprised me, because I had thought that dipoles were fairly narrow to their resonant frequencies, but I was getting much better performance than the W881 for airband and 2-metre work, and moreover was even receiving some SSB signals down around 7Mhz that the W881 could not hear.
Intrigued by this, I cut up more coathangers to provide two 53cm lengths (135MHz); I am not really certain, but I believe that this has improved matters even more (the London South VOLMET on 128.6 sounds clearer) - though I cannot put a figure on the improvement.
The formula for the total length of a the dipole is:
The factor of 0.96 is derived from a chart in the above book, determined by the ratio of the 2mm-thick coathanger wire to the length of the dipole; so, for 130MHz:
...and remember to leave a little extra for the bit you bend over and screw into the choc-block; I usually add about 7mm.
The antenna herein presented is very effective, very powerful (better for airband than either of the two commercial whips that I own) and very cheap, making it very attractive to people like myself who pursue their hobbies on a budget.
Given the simplicity of the design and the moderate ease of availability of parts (Can't find ethernet coax? Use TV feeder coax...) and of construction, I strongly recommend any scanner user to give it a go and see what your results are like.
I am currently looking for a (preferably UK-based) supplier from whom I can obtain good quality 50ohm or 75ohm 1:1 VHF/UHF baluns (also VHF 4:1 or 6:1 baluns for folded dipoles) fairly cheaply, suitable for soldering and project work experimenting with homemade dipoles attached to scanners; this would permit me to do some "balanced versus unbalanced" tests and lay that ghost to rest.