21-03-2018, 07:35 AM
Wayne
I use +12V up the coax to a "changeover" relay to switch between a vertical whip and a 10 metre long wire antenna (via an UN-UN) on my "house installation" {i.e. as different to my larger shack antenna install }.
The bias tee at the bottom is simply a pair of coax sockets with the centres joined via a RF-style DC blocking capacitor and with the output DC fed via a RF choke of about 10mH with a bypass capacitor to 'ground/shield' on the output of the on/off switch - plus a series resistor and LED to show when the power is fed up to the relay. All housed in a small jiffy box.. The DC for the bias tee / relay is sourced via a light fig-8 wire from the power supply for the transceiver.
The changeover relay box up top is a weatherproof ABS style containing a 10A 12V DPDT relay, another RF bypass capacitor, a diode wired across the relay coil, three stainless steel screws for output terminals and ground plus a SO239 for the coax connection in. It also houses the 9:1 UN-UN used to feed the longwire or whip, and in fact the DC feeds via the UN-UN (DC isolated secondary arrangement) before it hits the relay coil, diode and bypass capacitor. The otherwise unused relay contacts are used to 'ground' the unused antenna so that it acts as an additional counterpoise rather than as an active antenna via any relay contact-contact capacitance.
It all works very well and gives me some HF antenna flexibility with just one HF coax feed. So, yes, the bias tee idea really works.
I use +12V up the coax to a "changeover" relay to switch between a vertical whip and a 10 metre long wire antenna (via an UN-UN) on my "house installation" {i.e. as different to my larger shack antenna install }.
The bias tee at the bottom is simply a pair of coax sockets with the centres joined via a RF-style DC blocking capacitor and with the output DC fed via a RF choke of about 10mH with a bypass capacitor to 'ground/shield' on the output of the on/off switch - plus a series resistor and LED to show when the power is fed up to the relay. All housed in a small jiffy box.. The DC for the bias tee / relay is sourced via a light fig-8 wire from the power supply for the transceiver.
The changeover relay box up top is a weatherproof ABS style containing a 10A 12V DPDT relay, another RF bypass capacitor, a diode wired across the relay coil, three stainless steel screws for output terminals and ground plus a SO239 for the coax connection in. It also houses the 9:1 UN-UN used to feed the longwire or whip, and in fact the DC feeds via the UN-UN (DC isolated secondary arrangement) before it hits the relay coil, diode and bypass capacitor. The otherwise unused relay contacts are used to 'ground' the unused antenna so that it acts as an additional counterpoise rather than as an active antenna via any relay contact-contact capacitance.
It all works very well and gives me some HF antenna flexibility with just one HF coax feed. So, yes, the bias tee idea really works.
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
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http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.