A DAB Radio Antenna
I bought a DAB tuner to plug into the amplifier that drives the speakers near my desk. It came with a couple of bits of wire that terminated in an F plug; a combination described in the handbood as an 'antenna'. Ten minutes of precarious positioning gave me a barely-adequate signal which worked most of the time but would fade and break up badly from time to time. The electrical noise from several computers probably wasn't helping it. Some blu-tack held the wires in place for a while but it wasn't really tolerable for long.
Looking around the web I found a limited number of people selling commercial DAB antennas but they seemed rather pricey for what is only a few bits of wire, so I did a quick Google search for a Yagi calculator (just about every modern TV and radio antenna you see on buildings is based on the Yagi-Uda principles) and found this Yagi calculator - so I gave it a whirl. Aiming for a centre frequency of 220 Mhz it told me that I wanted 26cm between the elements, a director of 61.7cm, a radiator of 65.5cm and a reflector of 69.4cm.
Some old coathangers provided the wire for the elements and a broomhandle found in the garage was pressed into use as the boom. The coathanger wire didn't match any of my drill sizes, but using the next size up gave a loose fit that was made nice and tight by tapping a panel pin into the hole alongside the wire.
Traditionally the radiator of the yagi would be cut in the middle and the coaxial cable attached to each half, but that's not very convenient with the broomhandle method, so I adopted a more esoteric connection method - a 'gamma match'.
The gamma match involves connecting the braid of the coax cable to the middle of the radiator, then an 'L' shaped wire that connects into the inner of the coax and taps into one leg of the radiator element a magic distance up. This is actually a transmission line impedance transformer and to get it spot-on involves either tricky maths or the age-old cut-and-try approach. Instead of directly connecting to the radiator, a proper gamma match would use a capacitor (see later).
Opting for cut-and-try, I bent a loop at the short end of the L to hook onto the radiator, soldered my cable to the pins in the holes where the gamma match and radiator enter the boom and then slid the gamma match up and down until I got the best results. The over-long bit of the gamma protrudes from the far side of the beam and in this case seems to assist in getting the match right.
With the DAB radio connected to one end of the coax and the antenna waved around to find a signal it didn't take long to find a usable point for the gamma match to connect so I soldered it in place and tightened it up by hammering the pin further in.
The antenna is now up in my loft connected via some cheap 'low loss' tv cable courtesy of the previous resident. The DAB radio shows just about full-strength on its signal meter on all the multiplexes it has found.The antenna shows clear directivity - pointing it away from the strongest direction produces very noticable signal loss - and it's also obviously strongly polarised, rotatinge the elements to be parallel to the ground instead of vertical severely attenuates the signal. Overall, it's been a very succesful use of a couple of hours and some scrap material.
Having stuck it up in the loft I spent an hour or so building a NEC model to see what an antenna simulation program made of it. Rather than just model an idealised version, I measured mine (including the chunk of redundant gamma wire that I didn't bother to trim) and then modeled that instead. The photos below show the thing up in my loft and the calculated stuff is interesting to see. Tweaking the model lengths suggest that the extra bit of gamma match actually helps tune it correctly!
The model fed to NEC2 was generated from a perl program which generates the wire segment data, making it much easier to tweak element lengths.
CM DAB Yagi model CE GW 1 21 -0.26 -0.3125 0 -0.26 0.3125 0.000 0.001 GW 2 21 0.26 -0.35 0 0.26 0.35 0.000 0.001 GW 3 21 0 -0.33 0 0 0.0125 0.000 0.001 GW 4 21 0 0.0125 0 0 0.1275 0.000 0.001 GW 5 7 0 0.1275 0 0 0.33 0.000 0.001 GW 7 7 0 0.1275 0 0.02 0.1275 0.000 0.001 GW 8 7 0.02 0.1275 0 0.02 0.0125 0 0.001 GW 9 7 0.02 0.0125 0 0.02 -0.0825 0 0.001 GW 10 7 0 0.0125 0 0.02 0.0125 0 0.001 GE 0 FR 0 25 0 0 210 1 EK EX 0 10 4 0 1.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 NH 0 0 0 0 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 NE 0 60 30 1 -4.75000E-01 -7.75000E-01 5.00000E-02 5.00000E-02 5.00000E-02 0.00000E+00 RP 0 37 73 1000 0.00000E+00 0.00000E+00 5.00000E+00 5.00000E+00 0.00000E+00 0.00000E+00 ENUsing xnecview under Linux, here's what the output looks like in graph form (click the images for larger versions):
With way too much time on my hands (ok, time that I should have spent doing other things) I tweaked the simulation for something a bit more broadband. The Gamma match this time needed to be converted into the traditional version with the short leg of the L being replaced by a very small value capacitor. Here's the output of the simulation:
CM Broadband DAB Yagi model CE GW 1 21 -0.26 -0.3075 0 -0.26 0.3075 0.000 0.001 GW 2 21 0.26 -0.355 0 0.26 0.355 0.000 0.001 GW 3 21 0 -0.325 0 0 0.0125 0.000 0.001 GW 4 21 0 0.0125 0 0 0.0875 0.000 0.001 GW 5 7 0 0.0875 0 0 0.325 0.000 0.001 GW 7 7 0 0.0875 0 0.02 0.0875 0.000 0.001 GW 8 7 0.02 0.0875 0 0.02 0.0125 0 0.001 GW 10 7 0 0.0125 0 0.02 0.0125 0 0.001 GE 0 LD 0 7 4 4 0 0 5e-12 FR 0 21 0 0 210 1 EK EX 0 10 4 0 1.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 NH 0 0 0 0 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 NE 0 60 30 1 -4.75000E-01 -7.75000E-01 5.00000E-02 5.00000E-02 5.00000E-02 0.00000E+00 RP 0 37 73 1000 0.00000E+00 0.00000E+00 5.00000E+00 5.00000E+00 0.00000E+00 0.00000E+00 ENIn case it's not clear how the elements are positioned:
At the drive point, solder the braid of the coax to the driven element and the centre to the gamma match wire.