Kits & Modules
Geoff Theasby G8BMI has some thoughts on comparing antennas and introduces the Flowerpot from Australia.
Geoff Theasby G8BMI has some thoughts on comparing antennas and introduces the Flowerpot from Australia.
Any amateur who experiments with antennas eventually needs to compare them. This can be done with a VNA, which has the ability to save the results. The cheapest of these is still about £40, but here I describe cheaper means.
The first is the good old manual switch,
Fig. 1. Use a DPDT switch as a SPDT with the contacts wired in parallel to reduce losses, and use coax braid or heavy wiring. I have no means of checking the power handling ability, but it should be good for 50W of RF on the HF bands. I also have a Heathkit five-way switch, which has served me well for over 50 years. Internally, a wafer switch distributes the signals, and is rated for 1kW.
The second is an Easy TR Switch from Pacific Antenna, sold by QRP Kits for $20,
Fig. 2. This uses an electromechanical relay and is rated at 150W up to 50MHz, but I have successfully used it on the 2m band. It features an RF-sensing circuit, needs 12V at 25mA, and has been tested at 9V from a PP3, so it can be used in the garden without running a 12V DC line alongside the coax feeder. This little device allows two antennas to be compared while using only one feeder. There is a good downloadable eight-page informative guide.
Lastly, from Amazon, an HMC349 module, which is all solid-state, reputed to work up to 4GHz and costing about £14,
Fig. 3. It needs a supply of 5V at 4mA, and is matched to 50Ω at the RF input and the output, but limited to 33dBm (2W). It is ideal for switching between receivers.
TheVery Simplest Antenna
This could hardly be described as a kit, as it requires the absolute bare minimum of components, one! A 2m band antenna, which I made in 35 minutes, from sitting down at my bench to pressing ‘Transmit’.
Currently known as the Flowerpot antenna, it was developed by VK2ZOI (website below), building on previous work by VK3RYK and LA1AC, on the grounds that it could easily be concealed in a small bush or large indoor pot plant, where the overt presence of an antenna might be unwelcome. www.VK2ZOI.com
To make it, all that is required is 1m of braided screen coaxial cable, I used RG58. Remove 457mm of the screen, leave the inner insulator intact, and mark the cable 447mm below this point with marker pen, tape etc. Then wind nine turns of the cable into a choke balun, about 30mm in diameter, fastening securely. Job done!
The device may be suspended from a suitable point, or fixed into a plastic tube or conduit to make it self-supporting and as a former for the balun. I used 33mm white plastic kitchen waste pipe; white is less absorbent to radio waves than grey or black, Fig. 4.
Compared with a commercial 25W dummy load (SWR 1:1) it measures 1.2:1 at 145MHz, using my Diamond SX200 SWR/ Power meter. I tested it against my HB9CV using a homebrew switch (similar to that shown in Fig. 1), itself introducing a low SWR of 1.5:1, which gave similar results.
Fig. 1: Simple manual antenna switch.
Fig. 2: Easy TR Switch.
Fig. 3: The solid-state HMC349 module.
Fig. 4: Photo of ‘Flowerpot’ in kitchen waste pipe before final mounting.
This was built in a small plastic box, a metal one may be more efficient.
It can be made dual-band 145/432MHz by merely adding a coaxial sleeve at the mid-point. No electrical connection necessary. The antenna has been successfully scaled for other bands, even into the HF region.