TRAINING TOPIC: (Antenna Performance Issues)
Logan Storehouse ERC Training Topic for December 1, 2024
Rick Hughes, K7YPR
For this training session, the discussion will consider some technical issues that could affect the quality of your signal and your overall ability to communicate with others via Ham Radio. This specifically relates to antenna performance. One of the primary reasons for this net as well as others is to test and evaluate your ability to be heard by net control and other stations. Sometimes other factors such as unusual signal propagation, distance or terrain shielding may affect signal quality especially on Two Meters.
One measure of performance is an adequately tuned antenna which matches the impedance of your radio equipment. The standard we use for measuring that performance is SWR (Standing Wave Ratio). Similarly, the objective is to deliver all the power generated by the radio equipment to the antenna with none reflected back to the source. The point tonight is that minimum VSWR may not always be an indicator of antenna match. That may sound like technical heresy to some.
Here's the point: It’s possible for your antenna to be disconnected from the transmission line either loosely or completely and still show a relatively low VSWR at the transmitter end. That seems a little counterintuitive or contrary to conventional wisdom but here’s why: If there are standing waves on the transmission line, the measurement position along the line relative to the antenna load may give erroneous readings. The transmitter may be perfectly happy since all the output power is being absorbed in the transmission line in the form of heat and not the antenna with little power reflected to the transmitter.
This was the case with the antenna at the Providence EOC. Net control reported continuing weak signals from the EOC. A Bird 43 Wattmeter was connected to the transmitter output via a short interconnecting cable only to find minimal reflected power which raised the question as to what the actual problem could be. It turns out the city station’s Diamond X50 antenna, a popular choice of many hams, had a corroded connection between the radial pipe sleeve and the antenna input connector. The set screw which connects the radials to
the connector body was also loose. Diamond has since added a second set screw securing the radials. If you are using this antenna, make sure these connections are tight.
In this unusual case, a network analyzer was available and when connected to the antenna input at the transmitter end showed a periodic VSWR pattern down the transmission line to the antenna revealing an improper antenna termination. Because of the random and specific transmission line length, the transmitter input connection point along the line ended up at a low VSWR spot creating the illusion of a matched antenna.
To avoid similar problems, make sure all connecting surfaces are tight and those exposed are weatherproof. It may be necessary to periodically check various contact points which may become loose due to wind or temperature variations.
You might find the following YouTube videos instructive and worth watching:
Kristen McIntyre, K6WX, ARRL First Vice President Author of Ground is a Myth (2024 Hamvention) https://www.youtube.com/watch?v=VvE570NPmmI
“SWR? Who Cares!” A BARC (Benica Amateur Radio Club) Zoom Presentation July 14, 2020 https://www.youtube.com/watch?v=lqWhGctkusQ