Tag Archives: SWR curve

The 20m vertical – what was wrong with it?

I built a 20m vertical antenna for sota use. A squid pole supported the vertical section and the radials were elevated about 1.5m above ground.

When I first used it, results were not good. On receive, it sounded dead or near to it, with very little of the normal band noise I expect to hear on 20m. On transmit, the impedance was way out, an antennna matchbox being required to make it accept power from a radio designed for a 50 ohm load. Clearly something was wrong with the antenna.

It was simple enough. A quarter wavelength of wire attached to the side of a fibreglass squid pole. Three or four radials connected to the outer conductor of the feed line. The connections for the main radiator and the radials were on a plastic box. An SO239 socket was at the base of the box and another at the top for the vertical element.

Possible problems were considered. The connections in the box were simple enough. The length of the vertical element was checked. A quarter wavelength on 14.1 MHz, adjusted for a 95% length due to the wire thickness and the insulation calculated at 300/14.1/4*0.95 = 5053 mm.

I asked on the yahoo group SOTA_Australia whether anyone else had experienced detuning of wire elements laid along a squid pole, or had noticed losses introduced by a squid pole. Some comments were made by list members about their experiences – mainly good. Warren VK3BYD offered me some segments of the fibreglass from a broken squid pole, and I took up his offer. In a few days I received a Jiffy bag containing three segment of fibreglass tubing, probably the top and thinnest element of a squid pole.

At the Canberra Radio Club meeting in October or November I took the fibreglass segments along to do some experiments on the impact of the squid pole material on the losses in an inductor. Dimitris VK1SV brought his antenna analyser and a tuned circuit made from an air cored inductor and a tuning capacitor. Measuring the losses in a tuned circuit comprised of the inductor and a variable capacitor in parallel showed a very low resistance component of less than an ohm. Introducing various materials into the inductor core had various effects. The fibreglass sections from the broken squid pole had almost no effect. A piece of pine wood about 50 x 20mm and 100 mm long had a huge effect, the losses in the wood introducing many ohms of resistance into the circuit.

The focus then turned to the connections inside the antenna base, a box containing a few wires and sockets. I then discovered that at least one of the connections to the radial socket was not well soldered. I reconnected and resoldered all the connections between antenna, incoming feedline and the radials.

Next time I tried the antenna was at Spring Hill, the site of the original operation. This time performance was much better, with several European contacts made on CW and SSB, using the 10w from the IC703.

The frequency of best match was still not 14.1, tests indicated it was resonating lower than 14 MHz. The main element was electrically shortened by folding over about 100mm of the radiator at the top bringing the SWR curve of the antenna to a much more satisfactory level.

IMG_1401

Vertical antenna's base
Vertical antenna’s base

When used at Bowning Hill in a joint activation with Andrew VK1NAM, the antenna performed well, with the ATU not required. We made a number of contacts with European chasers.

Andrew Vk1NAM with radio equipment on Bowning hill 9/3/2014
Andrew Vk1NAM with radio equipment on Bowning hill 9/3/2014
Antennas - vertical on the left, linked dipole on the right
Antennas – vertical on the left, linked dipole on the right

The IC703 and IC706 have a very handy SWR curve feature allowing you to choose a band segment and measure the SWR at each of 3, 5, 7 or 9 spot frequencies, spaced by 10, 50, 100 or 500 khz, depending on the band and the frequency span you need. Using the 50khz increment centred on 14.250, with 9 spot frequencies allows for a quick check of how the antenna behaves across the band. I used that feature when adjusting the resonant frequency of a helical whip and wrote about it in an earlier post.

The outcome is that the antenna is now operating as it should and tests done on Bowning Hill comparing it with the linked dipole indicated that it is equally effective as a dipole at 7m above ground. When the dipole was end-on to the signals from the UK, there was a marked reduction in the signal level received at the EU end of the path.

The vertical is simple to erect and has no wind loading additional to the squid pole itself. For this reason I think it is a good option for SOTA operations.