Mike's Home Page

Disconnected jottings, random idling and odd thoughts

This will always be an incomplete work-in-progress; development continues on a desultory basis.

Contents
July '10
Ireland'05
Assorted Photos
September 09
Private photos
School Cruise 06
Jun '10
Home
Linux material
Dec 09
Sailing 2012
Other Photos
Radio
 > DAB Antenna
 > Emergency comms
 > GPS12XL Battery
 > Toaster Repair
Oct 09
Late 2004
Steam Organ Project
Feb '10
Ellie pics

Contact



This website is purely a personal effort by Mike Banahan of GBdirect. No warranty given for the accuracy or reasonableness of the contents is given or implied. This page last updated Sep 13 2006 12:05:52.

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!


DAB Antenna

Gamma match

DAB Antenna

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
EN
Using xnecview under Linux, here's what the output looks like in graph form (click the images for larger versions):

Gain/SWR

Polar @215MHz

Polar @220MHz

Polar @228MHz
In practice the antenna works extremely well but from the simulation it looks as if it's a bit narrowband and the element lengths perhaps a shade long (the peak gain appears at 212MHz). I'm not too bothered because I now get all my stations at near full-strength in an area that the BBC postcode calculator has as somewhere verging on marginal.

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:


Gain/SWR

Polar @215MHz

Polar @220MHz

Polar @228MHz
This is noticeably more broadband. The dimensions are: distance between elements 26cm, director 61.5cm, radiator 65cm, reflector 70cm. The gamma match consists of the coaxial cable braid soldered to the radiator 1.25cm from the centre (because of the broompole), a straight segment 8.75cm long parallel to and 2cm from the radiator, joined to the radiator at the far end through a 5pF capacitor. This very low value is usually obtained using flat tabs rather than wire, with the spacing adjusted for best results. An alternative is to use insulated wire and wrap it a few times round the radiator, again adjusting for the best response. Here's the NEC input file:
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
EN
In case it's not clear how the elements are positioned:
Antenna structure
At the drive point, solder the braid of the coax to the driven element and the centre to the gamma match wire.

Home