Here is a few photos of my working conditions at the moment. Basically, I’m operating from the rooftop of a 5 story apartment. My antenna is literally a piece of Bamboo with a bit of wire taped to it! Its roughly 1/4 on 15m, and it works a treat to be honest. Total cost was about $5! I don’t have a power source up there yet, so I’m at the mercy of both my laptop battery and the car battery running the transceiver. Click on the link below for some more photos.
Just thought I’d share some notes I’ve made regarding Antenna Theory. Believe it or not, it took many hours of scouring the web and consulting books such as the ARRL Antenna Handbook to come up with this list.
- Antenna Resonance is the point / frequency at which an antenna produces a purely resistive load. That is to say, there is no complex impedance component, eg 73 + j0 Ohms.
- Non Resonant antennas do not necessarily make bad performers, but care must be taken to ensure that matching components are correctly placed and efficient.
- Antenna Efficiency is directly related to how much ‘radiation resistance’ an antenna has. The shorter an antenna with respect to the driven frequency, the less radiation resistance it will have.
- Maximizing the current across the element (linear metal segment eg, whip) of an electrically short antenna will result in the greatest possible efficiency (all other things equal). This is achieved by correct placement of matching components.
- ‘Short’ Antennas (with respect to wavelength) present a capacitive impedance, they require the use of an inductor or ‘loading coil’ for matching. The optimal location electrically is at the top.
- ‘Long’ Antennas present an inductive impedance, they require the use of a capacitor or ‘condenser’ for matching. The optimal location electrically is at the top.
And that concludes the basics. Next time we’ll explore the affect matching has on antennas, some of the tradeoffs & other design considerations.
There is now a part 2 to this article which covers replacing the capacitors found in the antenna, if you’re more interested in that aspect of repair, please have a look here. Otherwise, read on for my solution to a broken Radome.
Background, Specifications & Waffle
At the start of my adventures into Amateur radio, I was lucky enough to be given all kinds of gear by the very friendly VK3HBN as an incentive to getting licensed – after all, who can sit by idly looking at lots of toys that they can’t play with?
One of these items was a lovely (read expensive) Diamond X510MA Dual Band Base Station Vertical Antenna. The specifications are as follows:
- 8.3 dB Gain on 2M (3x 5/8 Elements, Centre Loaded Co-Linear)
- 11.7 dB Gain on 70CM (5x 5/8 Elements, Centre Loaded Co-Linear)
- Max Power 200W FM (and more than likely SSB, remembering that phasing capacitors have Vpeak limits)
- 5.2 Metres tall with 3x 52cm Radials at feed point
- 3 Piece construction
There was however, but one catch
The antenna had suffered a slight mishap. Although details are scarce – perhaps to protect the reputations of third parties – the bottom section of the antenna had suffered a clean break as the consequence of a fall. While the antenna had previously been repaired by placing a section of 32mm PVC pipe over the break combined with some PVC Adhesive and ‘gaffer’ tape for sealing purposes, the repair had come loose and was looking somewhat worse for wear – I decided it was time for a complete overhaul. Details Below.