RX2 RF output?

WA2DVU
Posts: 43
Joined: Wed Nov 01, 2017 3:27 pm

RX2 RF output?

Postby WA2DVU » Thu Nov 30, 2017 4:42 pm

Am I correct in assuming that there is no way I can goof up and send RF out the RF2 jack? (I want to use my K9AY loop and have blown up 2 preamps over the years.) 73, Bill, WA2DVU
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w-u-2-o
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Re: RX2 RF output?

Postby w-u-2-o » Fri Dec 01, 2017 12:08 am

No way for RF to come out, but on some radios that input is grounded on transmit and some preamps don't like that. If you have a preamp like that it is cheap insurance to put a DC block in line with that connection.

73,

Scott
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w-u-2-o
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Re: RX2 RF output?

Postby w-u-2-o » Fri Dec 01, 2017 1:43 am

Example:

https://www.minicircuits.com/WebStore/dashboard.html?model=BLK-89-S%2B

Plenty more of those fish in the sea. Just Google "DC block".

73!
WA2DVU
Posts: 43
Joined: Wed Nov 01, 2017 3:27 pm

Re: RX2 RF output?

Postby WA2DVU » Fri Dec 01, 2017 2:17 am

Thank you for the no rf answer. My three rx only antennas - wellbrook loop, K9AY loop with LZ1AQ preamp, and a Miniwhip do not care about a shorted amp output to the rx. I have these three antennas going to a AIWA coax switch to my "rig saver" that I use to key my amp from what ever rig I am running.. The rig saver has a receive antenna relay that accepts the output of my coax switch and sends it to RX2. On transmit it disconnects and shorts rx line. Cool unit. 73, Bill, WA2DVU
K9RX
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Re: RX2 RF output?

Postby K9RX » Mon Dec 04, 2017 1:50 pm

CAUTION!

Just as a caution - Scott had suggested a DC Block inline with a RX preamp - however note that isn't going to work and in some/most/all cases will not prevent the preamp from being damaged if it is going to get damaged by having the jack grounded.

Adding a DC Block, a capacitor inline - is great if one wants to do a, what is referred to as a Bias-T addition to allow DC over the coax/feedline ... but that isn't what is going on here. The capacitor is, needs to be of necessity, a (near) 0 ohm impedance at AC .... and thus when one end of it is grounded the preamp still sees 0 impedance to ground (or close to it) so it could still get damaged. I was sure that was the case when I read this post - and confirmed it with my preamp maker - Lee at Hi-Z antennas.

the only thing that is going to prevent it is :
1) insert an external relay tied to the PTT line such that the output is disconnected (form A relay) or taken to a 50 ohm load (form C)
2) remove power from the preamp before the jack gets grounded
3) change the config of the "BPF" board internal to the radio... it is actually wired what I believe to be incorrectly. It has the input (external world) through the RX2 jack being grounded and leaves the input to the BPF floating! This causes two issues: a) the above described issue with a peramp and b) the possibility that you will still see too much RF in to RX2 from the internal floating BPF. I'd suggest the armature of the internal relay be swapped so that the BPF input (there is a LP there which it hits first) is instead grounded and the jack is either left floating or is tied to a high impedance - 100K or more.

Note for item 2 above at least in the 8000 I've not been able to determine when exactly the jack gets shorted... I THINK, or at least common sense would say it happens when one selects ADC2 as RX2 .... however I've tried that and don't always hear relays clicking in the process so I'm not sure what is going on. That stated it might still be a necessity, as it is in my case since my RX and TX antennas are close, to remove power from the preamp (I remove it from the complete RX array including the preamp) when TXing.

Gary
K9RX

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