Impedance of a colinear aerial

I dunno, but I get amazingly good reception of local digital TV with this antenna:

The free end is just a bit shorter than 1/4 wavelength. As you can see it is well guarded.

I have a Baofeng UV-5R for 144 MHz and 440 MHz, does FM too:

It's got to be frequency dependant, and if you pick the right lengths the impedance will be 50 ohms as the apropriate frequency.

It feels like an end-fed phased array to me... but testing is probably easier than doing this with pure thinks.

There's probably a way to get different results by using different impedances of co-ax too.

For what it's worth, my research indicates colinears (CoCos) are too complex for a "cookbook" calculation of radiation resistance of a CoCo in isolation. Instead, it's more practical to measure feedpoint resistance at resonance and assume most of it is radiation resistance. Please share you empirical data. Thank you.

The first attempt was a disaster.

A VNA showed impedances all over the place, restricting the frequency range to 50 to 250 Mc/s gave a Smith Chart like a kitten playing with a ball of wool. Compared with a vertical sleeve dipole, the signal was 20 to 30dB down.

Then I started reading the adverts for commercial versions and discovered that all of them had sloping radials near the bottm end. This made sense because I hadn't been able to understand where the 'bottom element' corresponding to the top whip was located; there should have been an earth plane or sleeve or something for it to react against.

Returning to the websites that had given me the initial information, I realised that they were written by web self-publicists who had copied genuine researched websites but copied them incorrectly. The'proper' websites showed that a sleeve or radials were needed.

Given this information, I came up with a second design:

I have listed the lengths in terms of wavelengths on the left and their actual measurements on the right. Sometimes there appear to be anomalies but these are due to some measurements having to account for the Velocity Ratio of the co-ax whilst others are adjusted for the end-effect.

The Smith Chart plot is still a mess and I haven't plucked up the courage to compare it with a dipole yet.

I have a feeling that such coax colinear arrays will always be a compromise because the vf of the coax shield inner is lower than the vf of the outer. In other words a coax with higher vf will reduce the compromise a bit ? Maybe a higher Zo coax helps?

I won't pretend to have a detailed understanding of this kind of aerial, but it seems to me that it should be able to work over only a narrow band of frequencies, say, no more than about 10% off w.r.t. the centre frequency, maybe even less.

Jeroen Belleman

Yes, the 2-metre band is 2 Mc/s wide, centred on 145 Mc/s, so we are looking at less than 0.5% each side.

There are alternative designs which use phase-shift networks instead of polarity reversals (they have non-inductively-wound coils of wire exactly half a wavelength long). They look a lot easier to make but introduce more frequency-dependent elements which would narrow the bandwidth and give even more ways it could go wrong.

I may be buying some better quality co-ax soon, to make a downlead with less loss than the present one. If I have enough to spare, I could try making another version of the aerial with that, as its Velocity Factor is likely to be a lot better.