Christmas Holiday Antenna

The Christmas Holiday Antennae - Feb 1995

The past month seemed busy with some of our members building antennae. The ZOI/DDB QTH needed a 70cm antenna. This had to be omni- directional, simple to build and tune-up, cheap and preferably with some gain to make up for both feed- line losses and to boost the 5 watt radio that we use on this band (a DSE Explorer).

A scan of the handbooks wasn't too produc- tive; quarter wave ground planes, dipoles, slim-jims etc seemed to be the only antennae, other than yagis, that were described. Armed with a calculator a few calculations were done which showed that a co-axial colinear would be ideal.

I recalled that there was such a unit de- scribed several years ago in one of the magazines but I couldn't remember which one nor much about it except that it was an 8 element and its overall length was about ten to twelve foot. Needless to say a search of my library failed to unearth the article so first principles.

The diagram shows the general arrangement. Each element is 221 cms. This is a half wave at 437 MHz allowing for the coax velocity factor and an additional 3% for end effects. The elements are soldered together with the inner of one element connecting to the outer of the adjacent element (and vice-versa). The collinear is mounted in a piece of white 25mm PVC conduit and one quarter wave below the bottom element a seven turn coil o f the feed- line is wound around the outside o f the conduit.

There is no tuning up required. Provided care is taken to keep the elements equal (say within 2 mm) and the joins are kept tight, this type of antenna will give a good match over a reasonable bandwidth. Unfortunately I do not have an SWR meter which gives a correct reading at 70cm but I did do some comparative measurements against other 70 cm antennae such as the rubber-ducky from a dual band hand held, a dipole cut for 70cm and the mobile 2m/70cm antenna. The collinear gave comparative readings across the 430 - 440 band.

In use the antenna appears to work OK; I am now doing the mod to the explorer to activate the upper Megahertz 439-440) so that we can access the Club repeater.

Bob VK2LAZ had a go at building the 2m mobile antenna described in January ARA. This is a half wave radiator with a "ringo" matching sec- tion at the base. Bob used a telescoping antenna rod rescued from an old TV set, a small piece of scrap brass flat, some spare co-ax (for the tuning capacitor) and a small off cut of wood to mount it on. The design is essentially ground plane independent and it was built so that it can be clipped onto the top of a car window. This allowed it to be moved from car to car and could be easily and quickly removed when necessary.

At the February mid-monthly, he brought it along so that It could be tuned. Once it was dis- covered that people standing around an antenna makes for difficulty in tuning, Bob's antenna was readily adjusted for a close to 1:1 VSWR. Anyone looking for a cheap, easy to build 2m antenna for portable/mobile use is referred to the ARA article