Lack of equipment commercially made for 4m is one of the challenges of the band. Having recently begun activity on 70MHz, I was looking for an effective antenna to use when out and about.

Like many other amateurs with a penchant for portable operations, I have bought and used several telescopic roach poles over the years. And it seems that every roach pole I’ve bought has added one more whippy top section to my collection of fibre glass that appears to be too thin to properly support an antenna.

So this project kills two birds with one stone: a four metre portable antenna that uses one of those spare fibreglass fishing pole sections.
The physical design is based on the many portable and mobile whip antennas available for both HF and VHF, with a durable, flexible vertical mounted directly onto a coaxial connector at the feed point. Its dimensions are determined by the materials available – the length of the fishing rod section being a key factor – and the tuning of the antenna.

Making the antenna
A 850 mm long top section from a telescopic fishing road can be used for the antenna’s vertical section. (Figure 1) Tubes of other, similar lengths can be used with adjustments made in the tuning step.

A one metre length of RG58 coax should be stripped and the braiding removed. The fibreglass tube can then be slipped inside the braiding and pulled down until 650mm of the fibreglass is covered. The braiding should be twisted at the rod’s tip and then trimmed off. Using solder to seal and secure this top end is a useful step, before stroking the braid down the fibreglass tube to produce an even, unwrinkled surface. Next, take a metre of insulated hook-up wire and strip its first 30mm. At the lower end of the braid bind the tube tightly with the bare wire and secure the joint physically and electrically by soldering the wire to the braid.

Wind the wire around the tube, creating ten turns over 120mm. Wind ten turns close-wound and then use tape to temporarily secure the end of the wire. Trim it where it passes the end of the fibreglass tube and strip 10mm.

Tuning the antenna
The antenna is tuned by moving the turns of the coil, either bunching the turns of the loose-wound section more closely together or separating turns from the close-wound coil.

SWR of 1.1:1 or less should be the target. On the prototype 1.1:1 at 70.3MHz was achieved, rising to 1.3:1 at 70.05 and at 70.5, using a MFJ 269.

Once the antenna is tuned it can be finished off.

Finishing
Make a small hole about 15mm from the bottom end of the tube using the end of a needle file or a small drill bit, so the free end of the coiled wire can be passed through into the tube’s central space. The bare wire end should be soldered to the coax connector’s centre pin. For most fishing road top-sections, a crimp type BNC connector (Figure 4) gives a good fit although other connector types could be used.

To attach the connector to the antenna, hot melt glue is introduced into the hole at the bottom of the rod section and the BNC connector pushed into the adhesive. I used a heat gun to warm the connector body and then to re-melt the glue before assembling the antenna. When fitting the co-ax connector, ensure that its centre pin is properly positioned and has locked into place. I decided on hot melt glue since using an epoxy adhesive would produce a more brittle assembly and epoxy won’t allow any further work once it has hardened.

The antenna’s tuning should now be rechecked and any final adjustments made. Once that is done, the tuning coil can be secured by covering it either with PVC tape or with heat-shrink sleeving slipped down from the top of the antenna.

Finally, a small plastic cap can be glued onto the antenna’s tip: I used the cap from a reel of RG58 coax secured with hot melt glue and finished with a short length of heat shrink, as shown in figure 5.