Some thoughts on a Vertical T antenna for 160M

Bob Whelan G3PJT

The simple vertical T antenna (Fig 1) is often recommended for 160m use because of low angle radiation (Fig 2) and a more realistic size than a full size 160M vertical. Any recommendation is couched in terms like ‘for 160 use your doublet with the feeders strapped, fed against ground through your ATU’. Such a casual approach is unlikely to lead to the best results. This short note suggests a better way.

Modelling a typical T antenna using EZNEC shows that a T erected at a typical height of between 10-20M and having a span of 30-40M will have a feed impedance, (R+-jX), between 4 - j190 to 23 + j150 ohms. These impedances mean that not only will some sort of antenna matching unit be need in order to match to 50 ohm coax but that losses due to ground quality and the system as a whole need to be minimised.

To put this in a practical perspective, my doublet was 17M high with a span of about 30M being the 450 ohm feeder and wire left over from an old antenna. Modelling this arrangement showed that I should expect an impedance at 1820kHz of around 11 + j16 ohms to which I needed to add some estimate of other losses.

Using TLA (ARRL Transmission Line Advanced programme) showed that such an impedance would need a L section shunt C of 2286pF and series L of 1.8uH to the antenna (Fig 3). However when I matched the antenna with this network, I found that a shunt C of 2850pF alone would give an excellent match at 1820kHz. Such a simple arrangement is very low loss.

So what was my actual antenna impedance? Using the program l_network.exe to perform the inverse calculation to TLA showed that my T antenna impedance was more like 14 + j22 ohms, which includes all losses of course. Now I don’t claim that my ground loss is as low as this might indicate ~ 3 ohms, but it did raise two questions in my mind.

What is the range of antenna impedances which only require a shunt capacitor to match to 50 ohms? And what would be the dimensions of such a T antenna?

TLA calculated that

R ohms jX ohms

5 14

10 19

20 24

30 24 would fill the bill.

To use these values as the basis in EZNEC you need to insert a load representative of your earth loss.

In my case and based on the above I estimated this at about 5 ohms and inserted a 5 ohm load in EZNEC.

More modelling suggested the following T antenna dimensions :

Height Span M

11 50

17.5 34

23 23.4

You might need to play around a bit with these dimensions or even adjust the end height of the span to trim things to the frequency you wish, and of course ground losses should be minimised and will affect these values too. But if you want to have a very efficient and simple matching arrangement as well as an effective DX antenna on 160M then these dimensions are the ones to go for.

References and websites.

EZNEC http://eznec.com

Transmission Line Advanced, TLA.exe is bundled with the ARRL Antenna Handbook available from RSGB Bookshop.

l_network.exe by R.J.Edwards G4FGQ (SK) can be downloaded from http://zerobeat.net/G4FGQ/index.htm

Fig. 1 T antenna

Fig. 2 Elevation radiation pattern

Fig. 3 L section matching