Amateur Antenna Assembling

Since getting back into amateur radio and also looking into SDR I've needed some antennas to really get the best out of both hobbies. My ground floor tenement flat is not conducive to good reception. Usually it's recommended that you make your own antennas at first so you can get an idea of how things work, learn some more about propagation, wavelengths, elements (driven, undriven and reflective) etc. - all pretty useful.

The best thing about antennas is that some of them are pretty simple. The kind of reception I was after - VHF and UHF mainly - can be done with some fairly relaxed tolerances and can be nothing more than a piece of wire with a few extra bits attached. Antennas for HF reception - which are limited to being quite large (unless you make a loop, which I may try next) - are beyond scope right now for me, as are satellite antennas (although I'm definitely trying that in the future).

The antennas below are for me to use indoors in my ground floor location to get better reception over that which a stock rubber duck antenna (walkie-talkie style, pictured here) can really achieve. They're longer, usually a half-wavelength instead of a quarter-wavelength, and this can improve both reception and your outgoing signal. They're also useful for me on SOTA expeditions - small outings to local hills and mountains to gain points for making contacts in an online-based competition. These better-quality and better-tuned antennas improve range and readability of signals and that's always the main point of the hobby really.

So, what are the kind of antennas I've made and what are they for? And, crucially, are they any good? Let's look at them all. Most of the home-made antennas were constructed by attaching them to the end of a length of RG58 coax cable (good enough at these frequencies) and terminated at a BNC plug. I have a BNC to SMA adapter to fit both the SDR dongle and my handheld radio - which technically loses you some signal gain, but not too much. The elements themselves are made of some cores from a standard mains cable. Thin enough to work with, thick enough to radiate well (which is important).

ADS-B Antennas

The good thing about ADS-B antennas is that they are really tiny. At the standard frequency of 1090 MHz a half-wavelength is 13.75 cm. A quarter-wavelength is therefore 6.875 cm. I took some major inspiration from this page for deciding what to make: https://lucsmall.com/2017/02/06/making-antennas-for-1090mhz-ads-b-aircraft-tracking/

The first thing I tried was a single length of wire stuck to the end of the coax and fed to the SDR dongle directly attached to my laptop. It wasn't great. For a start the USB ports in my laptop are quite loose apparently, so I had to wedge the dongle in place to keep it connected.

In terms of received signals I was getting one, maybe two planes every half hour or so? I live pretty close to an airport, which is both a blessing and a curse. At that location most planes are low to the ground. That, coupled with my ground floor location, severely compromised my ability to pull in decent signals. At this high a frequency the line-of-sight needs to be pretty good.

There was one, solid, full signal that I received for a good few minutes, and as a rare bonus the aircraft was displaying full positional information as well, allowing me to track its progress in the skies over Edinburgh. It appeared to be some kind of small Norwegian-registered air ambulance? https://abpic.co.uk/pictures/registration/LN-BSB

Now, the other reason this suffered was the lack of a ground plane. This is a reflecting element that simulates the ground and can either be the other side of a dipole antenna (you've seen them on the side of radio towers - T-shaped and fed in the middle) or some metal construction underneath the main element, such as a metal cone, radial metal rods in a sloping shape or even a tin can. I never actually made a dipole antenna for this frequency. It'd work out as the same wavelength (2 x 1/4) as a half-wave antenna, and I can do that much easier with a telescopic antenna. Dipoles are also much harder to construct and have more complex elements and tolerances.

Speaking of that telescopic antenna it comes in well here for the obvious reason that it can be adjusted to several lengths so I an use it for lots of things, but also because remarkably when fully closed up it measures exactly 13.75cm (up to the plastic bit, anyway - maybe there's more in there?). I've toyed with the idea of putting markings on the different segments to known lengths easier, but that's a job for a rainy day. I put it to use on the end of my SDR dongle which was in turn attached to the end of a USB cable to take the dongle further away from any sources of interference, particularly my laptop itself.

VHF antennas

The first thing I tried to do was make a VHF dipole antenna for use on the 2 metre amateur band. The most popular mode used in this band is a vertically-polarised FM signal. This is basically the normal type of FM voice (and sometimes data) transmission used elsewhere outside of the amateur bands for walkie-talkies, taxi and bus radios, pagers, and loads of other stuff. Vertically polarised antennas are much easier to erect than horizontal ones and usually the job requires nothing more than sticking it on the end of something and getting it high up. The dipole was literally just two pieces of wire cut to quarter-wavelengths of the middle of the amateur band (145 MHz), one soldered to the core and one to the shield.

Dipoles are slightly different beasts to vertical end-fed antennas given their signal radiation properties and construction, so the feedline needs to extend outwards for at least a quarter-wavelength to increase the quality of the radiated signal. I didn't realise this at first and just brought the coax downwards. You can see here how it was used on a nearby hill to activate this hill for SOTA.

Performance was orders of magnitude better than a rubber duck antenna, and I was able to get tens of miles with relatively low power. Sadly given the lack of structure to the elements I was unable to get it really high. I had to tape it to a fence post, as you can see.

I did later modify this design to add wooden rods (cut-down floor edging!) to bring the coax away properly and to hold the elements properly and add rigidity (in this picture pre-rods it's all just taped, but the coax is at least brought away to the side properly) but I don't possess the right kind of meter to measure how better that makes it. There's also the fact that I neglected to add the special bits that make a dipole work properly, namely bridging the gap between the two elements in the middle. It's all to do with impedance and I don't really understand it yet. But it worked very well indeed.

I mentioned that telescopic antenna earlier and I used that on the same hill. I seemed to get a much greater distance and a better readability of my signals over my inefficiently-made dipole. I was able to get higher by standing on top of things like big rocks and trig points, and this also improved the distance. I managed to make a contact with somebody in the Lake District on top of Helvellyn for a straight-line distance of 89 miles on 5 Watts of power.

UHF antennas

The same types of antennas can be used for UHF signals by modifying the element lengths to match the wavelength required. The higher frequency and shorter wavelength of UHF signals means that line-of-sight starts to get more important as those signals won't get bent as much by buildings or the surrounding landscape. Therefore I wanted to do something to improve the gain of my outgoing signal as well as make receiving of weaker signals possible.

I chose to make a type of directional antenna called a Yagi - much easier to do with element lengths this short. It features multiple undriven (not attached to the radio) elements of varying lengths to "focus" a signal and a reflector at the back to help even more. The driven element in the middle is basically like a dipole.

Again using the wooden floor edging as a framework for the antenna I cut some wires and wood to the lengths specified with an online calculator. As well as the elements themselves the distances between them also need careful measurement. Again, I chose the centre of activity for the amateur band here, 433 MHz. I also tried to implement the impedance-matching section of the Yagi antenna - something I hadn't bothered with on the dipole. I cut another small section (5 cm) of wire and attached it across the two driven elements in the middle. That... does something. Helps to stop power coming back down the antenna. One day I’ll learn that properly, but for now I'm just pissing about so I'm not too bothered.

The first thing to do was to demonstrate that it was at least made properly and giving gain in the proper directional pattern. So I aimed the antenna in the general direction of the nearest repeater, moved the antenna around, and checked to see if the signal went up and down. And it did, massively. And with a handheld you can talk with one hand and aim the antenna with the other.

A major problem is that nobody uses UHF analogue anymore, though. Digital radio is popular, but I have no gear for it. So there was little point to this other than as an exercise and it went well. I could use it for other UHF frequencies though on the SDR or for scanning. Not perfect, but better than a rubber duck and the gain may be required sometime.

Where next?

I was really chuffed with how all of this turned out. Having never done this before I was surprised how easy it can be to make something useful. Of course you can make them as complex or as involved as you want, improve the materials, the construction methods, etc. Lots of people use PVC pipe to hold their elements and cap the ends, making things somewhat waterproof.

In terms of future steps it seems the best thing to do is to make a good quality vertical with a ground plane and get it high up on a fibreglass mast. It's easier to handle than a dipole and the performance is much the same. Can't see the point of dipoles unless you're using a yagi for the gain. Much easier to bung a telescope on the end of a bit of cable and get it nice and high. So I'll be looking for some pipe to do this. Unfortunately it'll be a metre long so I need a way to make it out of sections for easy packing. Detachable ground rods too would be a bonus.

ADS-B seems to be out unless I can get a decent position for a receiver. One day... And UHF? Meh.

Thing is, there isn't a "best" antenna because they're all for different things. No ground plane may be ok if I'm in a field. A Yagi may work if I can mount it to the side of something and aim it easily. And the dipole... actually I'm struggling to see why they're useful unless in Yagi configuration, and the angle of radiation isn't great for distance. Am I missing something? They have a specific lobe pattern you may want, I guess.

Lots to think about, then. And a long-term goal of making a nice travel-capable 2M portable antenna. Probably a ground plane with some removable elements.