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Johnson T-R switch receive port output levels?
I have acquired a Johnson T-R switch and during testing at 3.5+ MHz. and
about 100W into a good dummy load, with the receive port terminated with 51 ohms I'm seeing an initial level on the order of four volts peak before the 6BL7 shuts off and reduces the level. Is this normal? If not, what is normal? Thanks. 73, Bob AD3K |
Johnson T-R switch receive port output levels?
On Aug 20, 9:37*am, "Bob Spooner" wrote:
I have acquired a Johnson T-R switch and during testing at 3.5+ MHz. and about 100W into a good dummy load, with the receive port terminated with 51 ohms I'm seeing an initial level on the order of four volts peak before the 6BL7 shuts off and reduces the level. *Is this normal? If not, what is normal? Thanks. 73, Bob *AD3K Bob,I haven`t used one of these in years ,But, I think that 4 volts at rec terminals would be acceptable ..At 4 volts on rec terminals not enough current would flow to do any damage to the input circuit..High voltage to rec terminals ,like lightning surges etc are the ones that do damage.. GL Harold W4PQW |
Johnson T-R switch receive port output levels?
Bob,I haven`t used one of these in years ,But, I think that 4 volts at
rec terminals would be acceptable ..At 4 volts on rec terminals not enough current would flow to do any damage to the input circuit..High voltage to rec terminals ,like lightning surges etc are the ones that do damage.. GL Harold W4PQW Harold, Thanks for your reply. It puts things in perspective. 73, Bob AD3K |
Johnson T-R switch receive port output levels?
"Richard Knoppow" wrote in message
... Are we talking about RF here? Yes. I was triggering the scope on the transmit RF and looking at the RF on the output of the receive port across 51 ohms. 73, Bob AD3K |
Johnson T-R switch receive port output levels?
Bob Spooner wrote:
"Richard Knoppow" wrote in message ... Are we talking about RF here? Yes. I was triggering the scope on the transmit RF and looking at the RF on the output of the receive port across 51 ohms. 73, Bob AD3K That about +16dBm. Shouldn't do any real harm. Healthy signal but non-destructive. |
Johnson T-R switch receive port output levels?
Bob Spooner wrote:
I have acquired a Johnson T-R switch and during testing at 3.5+ MHz. and about 100W into a good dummy load, with the receive port terminated with 51 ohms I'm seeing an initial level on the order of four volts peak before the 6BL7 shuts off and reduces the level. Is this normal? If not, what is normal? Thanks. 73, Bob AD3K Now that you're reassured about receiver safety, please let me know about your experiences with that Old-timey switch. Years ago, I used a similar one with my Hallicrafters HT-37 and Drake 2B. I found that the Xmtr's resonant tank circuit produced a null that went through the T-R switch, and pulled down the 2B's sensitivity. I eventually abandoned the Electronic switch for a relay. Jerry, W9NPI |
Johnson T-R switch receive port output levels?
"afcsman" wrote in message . .. Now that you're reassured about receiver safety, please let me know about your experiences with that Old-timey switch. Years ago, I used a similar one with my Hallicrafters HT-37 and Drake 2B. I found that the Xmtr's resonant tank circuit produced a null that went through the T-R switch, and pulled down the 2B's sensitivity. I eventually abandoned the Electronic switch for a relay. Jerry, W9NPI Jerry, I've seen references to an article in QST about changing the length of the coax between the transmitter and the switch when that happens. I used a homebrew version of the T-R switch in the early to mid 70's with a Valiant II and HQ-170A and never noticed a problem. But then I don't remember ever checking to see if the receive sensitivity went up when I detuned the tank circuit. If I run accross the information about which issue contains the article giving the analysis, I'll post it here. One thing that can be a problem when using the T-R switch with modern amplifiers that aren't class C is that amplifier noise will get into the receiver if the amp isn't biased off during receive. I've seen this on the scope while doing tests using my JST-245. I'm planning to use it with a Collins 310B, so noise shouldn't be a problem. 73, Bob AD3K |
Johnson T-R switch receive port output levels?
Bob Spooner wrote: I have acquired a Johnson T-R switch snip Hi, Don't forget to include a low-pass filter between the electronic TR switch and the antenna. I got c.w. thumps in my stereo system when I tried operating without a filter. I assume TVI would be a problem without a filter, also. The Johnson TR switch also works well when placed between a separate receive antenna and the receiver. It actually has a good noise figure, and provides some receive gain. It works very nicely with a Collins 51S-1, due to its 50 Ohm output to the receiver. The 51S-1 requires a 50 Ohm antenna for its front end tuned circuits to work as designed, so a random length receive antenna can provide disappointing operation otherwise. Be careful about burning your hand - the Johnson TR switch runs hot! 73, Ed Knobloch |
Johnson T-R switch receive port output levels?
I vaguely recall that some T/R electronic switches added the bandswitch to
eliminate suckout. 73, Colin K7FM |
Johnson T-R switch receive port output levels?
Ive seen it proposed that you connect to the valve anode .via a high
voltage cap ...never tried . looked a bit risky with 3/4 kv of rf .. but i suppose that would prevent the 'suck out' from the pi tank cct .. the rx would be looking through the cct , as long as the vlave was biased off .. not too much noise ? G .. On Aug 23, 1:15*am, "COLIN LAMB" wrote: I vaguely recall that some T/R electronic switches added the bandswitch to eliminate suckout. 73, *Colin *K7FM |
Johnson T-R switch receive port output levels?
On Wed, 27 Aug 2008, Graham wrote:
Ive seen it proposed that you connect to the valve anode .via a high voltage cap ...never tried . looked a bit risky with 3/4 kv of rf .. but i suppose that would prevent the 'suck out' from the pi tank cct .. the rx would be looking through the cct , as long as the vlave was biased off .. not too much noise ? G .. I thought that was the common form, certainly in homebrew articles that seems to predominate. Yet, the commercial ones are likely to be standalone, so one doesn't have to fuss with the transmitter. I've never used one, and never given it much thought. I thought the suckout issue was when it was connected to the plate of the transmitter, but that's more likely because the suckout issue was a common issue with TR switches, and I was more familiar with connecting it to the plate of the transmitter's output tube. One thing, it certainly was not uncommon to see articles where the TR switch included a stage of amplification and a tuned circuit of some sort. Also, when connecting to the plate, they'd use a very small value capacitor, and one article I just checked said to use an even smaller value for a higher power transmitter. That's going to cause some sort of loss, indeed that article mentioned one might need to add a tuned circuit to the TR switch to compensate for the loss. MIchael VE2BVW |
Johnson T-R switch receive port output levels?
On Aug 27, 5:19*pm, Michael Black wrote:
On Wed, 27 Aug 2008, Graham wrote: Ive seen it proposed that you connect to the valve anode .via a high voltage cap ...never tried . looked a bit risky with 3/4 kv of rf .. but i suppose that would prevent the 'suck out' from the pi tank cct .. the rx would be looking through the cct , as long as the vlave was biased off .. not too much noise ? G .. I thought that was the common form, certainly in homebrew articles that seems to predominate. *Yet, the commercial ones are likely to be standalone, so one doesn't have to fuss with the transmitter. I've never used one, and never given it much thought. *I thought the suckout issue was when it was connected to the plate of the transmitter, but that's more likely because the suckout issue was a common issue with TR switches, and I was more familiar with connecting it to the plate of the transmitter's output tube. One thing, it certainly was not uncommon to see articles where the TR switch included a stage of amplification and a tuned circuit of some sort. Also, when connecting to the plate, they'd use a very small value capacitor, and one article I just checked said to use an even smaller value for a higher power transmitter. *That's going to cause some sort of loss, indeed that article mentioned one might need to add a tuned circuit to the TR switch to compensate for the loss. * * MIchael *VE2BVW Yes , that was as far as I went , made a dual gate fet t/r switch, but it needed a tuned cct , did work on the low z side but was down in gain ... G .. |
Johnson T-R switch receive port output levels?
"k3hvg" wrote in message . .. Bob Spooner wrote: "Richard Knoppow" wrote in message ... Are we talking about RF here? Yes. I was triggering the scope on the transmit RF and looking at the RF on the output of the receive port across 51 ohms. 73, Bob AD3K That about +16dBm. Shouldn't do any real harm. Healthy signal but non-destructive. Electronic switches might not have the isolation you would prefer. With a single band you have the luxury of hanging a stub on the RX port to short the input during TX Otherwise, something should do that fast enough. Since there is 100w pep and I assume you measure 4v pp that is 16/51= .314 w pep so: 10 log 100/.314 = 25 dB isolation. Not real swift. I would want better than 35 dB Isolation. Even more for QSK. I wouldn't want to wait for the RX to recover. I have equipment that burns out the input protection above ..25 w input (ave.) You might not have trouble with a tube RF amp but... |
Johnson T-R switch receive port output levels?
"JB" wrote in message news:fuAtk.914$UX.763@trnddc03... Electronic switches might not have the isolation you would prefer. With a single band you have the luxury of hanging a stub on the RX port to short the input during TX Otherwise, something should do that fast enough. Since there is 100w pep and I assume you measure 4v pp that is 16/51= .314 w pep so: 10 log 100/.314 = 25 dB isolation. Not real swift. I would want better than 35 dB Isolation. Even more for QSK. I wouldn't want to wait for the RX to recover. I have equipment that burns out the input protection above .25 w input (ave.) You might not have trouble with a tube RF amp but... The problem with a stub is that you still have to disconnect it somehow during receive, right? I want to work full break in without listening to a mechanical relay. From my tests, I've found that the T-R switch attenuation isn't linear - higher power levels turn the switch off faster and produce more isolation than low power levels, as would be expected with an active device that used a sample of the output signal to drive a grid negative for switching. My old Valiant II had an output of 275W for CW and isolation was no problem for the HQ-170A receiver. I could hear my transmitted signal and tell whether it was clean or not. Of course, the AGC on the 170A might be better than the older generation NC-125 and NC-173 I'm planning to use. The T-R switch is rated for up to 4KW, so it was just loafing even with 275W. 73, Bob AD3K |
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