Front End Protection

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How does the antenna switching work between Transceiver Antenna Port 1, SCU-A RF Input, and SCU-A RX Loop Output? I have the 6700 so there is also SCU-B.

What is the maximum signal before damage will happen? What components are at most risk? Can front-end protection be added in the "loop" area before the components most in danger?

Understanding high power band-pass filters is the normal but they will not protect from two stations on the same band (CW and Voice or Digital) on beams pointed at each other. The thought of blowing the SCU digitizer is scary.
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Stan - VA7NF

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Posted 5 years ago

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Al K0VM, Elmer

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Stan,
see this previous discussion ..
http://community.flexradio.com/flexra...

AL, K0VM
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Stan - VA7NF

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Thanks Al for the reference - didn't find it on a search.

That leaves the other half - Can a potentially high signal on Ant-1 get fed to SCU-A RX Loop Output, have protection there, then fed back into SCU-A RF Input for QSK receive?
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Steve - N5AC, VP Engineering / CTO

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Official Response
Overload is at +9dBm on both SCUs and protection trips in at about +17dBm as I recall. A signal at overload will not damage the ADC, but it is the start of distortion from being too large. Damage occurs at a level above +20dBm, but the radio is designed to disconnect the antenna before this occurs (probably shouldn't test this at home though ;-) ). The path from ANT1 through RXA-OUT has no protection to speak of -- you are primarily going through relays to get to the SCU0 input. The protection is on the RXA-IN before it gets to the ADC in SCU0.
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Tim - W4TME, Customer Experience Manager

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"Damage occurs at a level above +20dBm, but the radio is designed to disconnect the antenna before this occurs (probably shouldn't test this at home though ;-) )"

I inadvertently tested the disconnect protection with a series of near lightning strikes last spring. The protection relays chattered for a second. The radio is still working great.
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Stan - VA7NF

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Thanks Steve for the information.

So that means, if I insert an Array Solutions AS-RXFEP between RXA-OUT and RXA-IN, also set the RX antenna to RXA-IN that this device will add to the protection with no damage between ANT-1 and RXA-OUT.
The protection device will also, due to the "softening" of the RF peaks, probably increase the IMD during these brief periods of high input RF.

I suspect QSK may suffer. Will it keep up to 30 WPM such as used in contests or should QSK be turned off?
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Steve - N5AC, VP Engineering / CTO

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First, Graham is much more knowledgable about the effects of a device like this than me so we'll see if he chimes in. My thinking is that if you are going to be seeing RF over +9dBm REGULARLY then I would not add a device like this. By effectively clipping RF signals, you will add the harmonics of that signal into the RF as you reshape the waveform. A better solution is probably an attenuator or a filter for the specific signal that you see that will go over +9dBm. On the other hand, if your goal is to protect against sudden bursts of RF, this device is OK -- but I'd be interested in where you think this RF is coming from. My first thought is lightning and this device placed where you suggest is not going to help you with lightning -- you would be better off placing something between the antenna port and the radio. In short, I'd want to understand what scenario you are trying to protect against before making a judgement, but off the top of my head, I don't see a good scenario for this device.
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Stan - VA7NF

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The environment I'm looking at is ARRL Field Day where there will be 4 stations with multiple antennas within a few feet of each other. Normal safe practice is to have band pass filters for each band but it is possible for the GOTA (Get on the Air) station to fire up on the same band or the option to put two transmitters on the same band (CW and SSB) in close proximity to the antenna the 6700 is on. Safety first!
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Tim - W4TME, Customer Experience Manager

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Stan, let me give you a real world example of the stoutness of the FLEX-6000 front end. At Pacificon 2013 we used an outside magnetic receiving loop connected to the FLEX-6000. Less that 120' away was a 1500w special event station. While it was transmitting on 20m, we were able to receive moderately weak signals 50-70 kHz away. The special event station did not "desense" the receiver.

I do not think you are going to have a problem on FD, especially if all stations are under 150w to get the extra point multiplier.
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Graham / KE9H

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Stan:

The circuit as in the AS Front End Protection accessory will start clipping some ten dB before you reach overload in the Flex-6000. In other words it will decrease the overload capability of the radio by some 10 dB. The equivalent circuit is already in the radio, but set up to not distort the incoming signal until a higher level is reached. As Steve said, the radio fully operates up until overload around +9 dBm. Going into data converter overload doesn't damage anything, just causes distortion. Electronic clamping (as provided by the AS-FEP) will occur a little above this, and a relay will disconnect the signal input above +24 dBm.

Note that an AS-FEP on the RX-A loop will only protect SCU0, you would need a second one on RXB loop to get full input protection on a 6700.

As long as what you are worried about is just strong signals coupling in from nearby antennas, I think you are better off using the 6700 as is.

We can't guarantee that you couldn't come up with a situation that would hurt the input of the receivers, such as directly keying a high power transmitter into the antenna input connector, but what is there is pretty well protected.

73's
--- Graham / KE9H

==
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K9SO

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My new Beverage is a fantastic antenna, but unfortunately I had to run it within 100 foot
of my vertical transmit antenna. I can run it from one of the transmit ports OK (which is dangerous for the Beverage) or via the RXA port on the 6500. The 6500 immediately goes into protect mode at 1KW levels. I'm clearly above the +24 dBm protect limit Grahm mentions in this thread (Thanks to the engineering team for protecting me from myself.)

My question is if anyone has had experience with the Array Solutions AX-RXFEP protect module when the antenna coupling is high enough to go into protect mode without it? It seems simple enough with protection coming from back to back 1N914 diodes and per spec limits the power to +10 dBm.

Alternatively, I could use the more expensive DX Engineering solution that essentially uses relay switching to disconnect the RX antenna. Since I'm running 30 WPM full break-in CW, I'd rather not add another relay into the equation. 

Anyone with experience with these solutions?

73,
Fred
K9SO
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Ria - N2RJ, Elmer

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I use the DXE RG5000HD. Works pretty well.
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K9SO

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Ria,

I received the RG5000HD this morning and it completely solved the problem I was having with the RXA port shutting things down.

Thanks for the recommendation.

73,
Fred
K9SO
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bjn644 .

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Same problem here. After spending the the day stomping around in a couple inches of snow installing my new beverage on the ground antenna, I find out if I run any power on 160 the 6400 goes into protection mode. I find it discouraging that the RX port can't be isolated enough without having to buy a $90 aftermarket gadget. I saw on another post about a power calculation chart, but that link is bad. Is there possible some setting I'm missing? I tried it in FDX mode and the same thing happens.
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K9SO

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I ended up putting a small relay in a box that shorts out the antenna whenever PTT is activated. I think that's the best option for you if you can't physically separate the TX and RX antennas enough. 

I did not like the diode clamping solutions. Even though it solved the shut down issue, it's a non-linear device being installed in your RX line. I suffered through all kinds of intermods from AM broadcast stations over 20 miles away even with the "high power version". I then added an AM band reject filter, but still had problems with it. 

73,
Fred
K9SO