TNC-Adpater to for optional antenna mounting position

[FlightSim]

Hi I was going to use a TNC Adapter to link antenna's to a Radio so that I have the choice of two antenna mounting positions. TNC directly on top of the system/box for 2.4db whip antenna and underneath the system/box for a secdondary to attach a 1 meter cable to, for a larger antenna such as a 6db collinear antenna. Is this a bad idea for any reason, will this be a problem for attenuation/imepdance etc. Or are there any other condiserations needed to be taken into account. Thanks Paul

 

[FlightSim]

And its a TNC T-Adapter I was refereing to for linking the Radio to the two different antenna positions. Thanks Paul

 

[VEBill]

Your description isn't perfectly clear, so this response is based on what I think you're saying. The RF cable will pass through a T-connector for position 1, and then carry on to position 2.

The issue isn't the T-connector, it's the significant length of unterminated transmission line hanging off when position 1 is in use. That'll cause reflections, frequency / length dependent cancellations, VSWR, etc.

So no.

 

[VEBill]

"...Is this a bad idea?"

I mean: yes.

 

[VEBill]

Unless, if the system is used at just one frequency, then you could tune the length of the unterminated transmission line to present effectively an open circuit at the T.

Note - the opposite case of the open T is not usually an issue because it's such a short length of unterminate transmission line that it would present an open circuit right up to extremely high frequencies, corresponding to its 1cm length.

 

[FlightSim]

If the secondary antenna with the 1 meter cable wasn't in use this would be disconnected from the bottom of the system/box that it attaches to. Within the box there would be a TNC T-adapter on a short lead of say 5 cm to the radio then the TNC cables to primary and secondary TNC Antenna connector are spurred of from the TNC T-adapter to the top TNC connector which would be about 20 cm and the same length again (about 20cm) from the T-Adapter to the TNC on the bottom of the system/box. The application of the Antenna's is for reception of RTK corrections from a base station. So when either is used that leaves unterminated line of about 20cm. I take it that causes some kind of impedance mismatch that causes reflections/attenuation, but at this sort of length will the effects or degradation on performance still be significant. Thanks Paul

 

[FlightSim]

The radio would be used for reception only but not at just a single frequency. It would be used at any frequency from 450 to 470 MHz (UHF 2). Thanks Paul

 

[VEBill]

Those numbers (20cm, 450-470MHz) aren't too far off the worst case.

You could use an RF switch or relay.

You could install a loop-in connector. Out, jumper, back in and out the other side.

 

[FlightSim]

Are TNC swtiches easily available could only see one's for coax, and is there relays avaialbe in TNC type. The jumper sounds like a great idea. So from what your saying I just have a 3 two connectors on the base of the box, one is directly connected to the TNC connector on the top of the box for the 2.4db Whip antenna, then the second TNC connector on the base of the box is the loop-in connector. So to use the TNC 2.4db I just use a short jumper to link the two connectors. Then if I use the secondary antenna via the 1 meter cable I just connect straight to the Radio TNC Connector on the base of the Box. The jumper method has quite a few TNC connectors for the use of the 2.4 db antenna mounted on top of the box, is this bad for attenuation (2.4db antenna - TNC to box base 2.4db Antenna TNC - TNC (Jumper) TNC - TNC box base Radio to radio. Thanks Paul

 

[VEBill]

Loss through connectors has typically been exaggerated. Loss through a TNC or BNC connector at ~460MHz would be negligible, 0.0-something dB.

Switches or relays with (for example) BNC connectors would be more common than TNC. One could mix and match through the signal path. Just because you TNC on the external connectors doesn't mean you can't use other connectors inside.

Or, at such UHF frequencies, one might be able to carefully wire-up a normal low voltage switch, using skillful RF wiring techniques... ...maybe, it's at the hairy edge. But hobbiests were playing with UHF many decades ago using skillfully applied point-to-point chassis wiring and vacuum tubes.

If you space the Out & In connectors just right, then you might be able to make an all-adapter jumper using two right-angle, and one double-male, adapters. Beware this spacing may slightly vary with exact PN of specific adapters used.

eBay has some good suppliers of connectors, adapters, etc.