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Hi, I have made an indoor active broadband loop for HFusing the LAMB-1B amp board from Casarain.
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Having now got back into amateur radio I want to know if there is anything I can build on the lead to protect against accidental transmission from my rig.
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If someone could point me in the right direction of a simple circuit Id be much appreciated. Thks. Adam
M6RDP?
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Hi Tom, it's an Icom IC-737?
73 Adam
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Hi
Use ptt out ( used to switch amp to tx)to control a little relay in line with rx coax. If the relay goes after bias T ( loop side) it will also shut off power to loop preamp.
The relay crt is dead easy, use the power for bias t to supply crt.
Or You could just use 1transister inline with power to bias t, this also controlled by ptt. That would just kill power to loop on tx.
However if you are using antenna 2 out in rig and not rx in, then you can easy blow it up if you tx from ant 2 out into loop..so if you can always use rx in antenna socket.
Simon g0zen
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Looked up the amp. The two transistors seem to be marked as "1A" which are generic 2N3904 surface mounts. They would be National Semi process process 23 devices which have a 2db NF above 100Khz. One of the documented transistors of the late 1960's. If you pop a few, they are easy to replace and inexpensive. 2N3904A's are the same device. The "A"s seemed a bit more consistent, box to box.
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Almost any relay "cure" is going to be more costly than the current online price of the of the amp (the LAMB-1B amps seem to be on sale at the Casarain site right now). Even a PIN diode in reverse parallel to a Schottky diode to drive the PIN will cost more than the amp. (This was the cure for protecting the 20mA input fuses on our AIL test sets). >"AIL" is Airborne Instrument Labs<.
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If you can find them, series #49 lamps between the loop and amp inputs will also work as inexpensive 50 ma fuses. Small cold resistance but large (18X and fast) resistance hot.?
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Jim/VEZ
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Add a PIN limiter diode across the input terminals. Why a PIN diode and not an ordinary small signal diode like 1N4148? The former's longer carrier lifetime prevents the diode from "following" the RF signal; i.e. being turned on and off and producing intermodulation products. A good reference is this: . Referring to fig. 1 in the referenced article, the shunt inductor can be omitted as the loop plays the same function.
73, Leong, 9M2LCL, ex-9W2LC
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Thank you everyone who has helped with my question for these great ideas. I'm pretty sure I understand how I need to proceed now. The biggest problem with the Icom 737 is the pesky transmit button just a few millimeters underneath the on and off switch. I have already had accidental activation of TX. Made just be wiser not to combine the two aspects of the hobby (SWL and TX) on the one radio.?
On Sat, 22 Feb 2025 at 4:48, biastee via groups.io <biastee@...> wrote: Add a PIN limiter diode across the input terminals. Why a PIN diode and not an ordinary small signal diode like 1N4148? The former's longer carrier lifetime prevents the diode from "following" the RF signal; i.e. being turned on and off and producing intermodulation products. A good reference is this: . Referring to fig. 1 in the referenced article, the shunt inductor can be omitted as the loop plays the same function.
73, Leong, 9M2LCL, ex-9W2LC
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Hi
Use the rx in antenna port for loop..its an icom so should have one.
(If its txing into loop you worried about.)
Simon g0zen
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It will be informative to follow the link below:
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Have a nice day!
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Jim/VEZ
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Jim, thank you. It's always easier to follow a diagram. Much appreciated
On Thu, 27 Feb 2025 at 3:13, James Redding WA9VEZ via groups.io <wa9vez@...> wrote: ?
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It will be informative to follow the link below:
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Have a nice day!
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Jim/VEZ
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Hi Jim,
I wonder if a 1n4148 and a 1n4007 would work in the same way for HF?
Andrew VK5CV
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From: loopantennas@groups.io <loopantennas@groups.io> on behalf of Adam via groups.io <adamtoynton@...>
Sent: Thursday, 27 February 2025 6:14:22 pm
To: loopantennas@groups.io <loopantennas@groups.io>
Subject: Re: [loopantennas] TX protection on RX Loop
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Jim, thank you. It's always easier to follow a diagram. Much appreciated
Adam
On Thu, 27 Feb 2025 at 3:13, James Redding WA9VEZ via groups.io
<wa9vez@...> wrote:
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It will be informative to follow the link below:
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Have a nice day!
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Jim/VEZ
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Hi Andrew,
The 1N4148 ist a fast switching Diode. Some 1N4007 have a long carrier lifetime and behave like a HF PIN Diode.
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regards, Fred
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Are we maybe overthinking this ?
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A pair of back to back 1N4148's or a BAV99 should be perfectly adequate in a receive situation.
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I use a pair of 1N4148's across the antenna cable from my "old school" external car radio antenna, that feeds the broadcast radio, as I also use a roof mounted screwdriver HF transmit antenna that is less than a metre away, and I've not had any problems from the 100w of RF that is being radiated.
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Regards,
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Martin
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On Thu, Feb 27, 2025 at 10:44 AM, Martin - Southwest UK wrote:
A pair of back to back 1N4148's or a BAV99 should be perfectly adequate in a receive situation
It depends. With an E-field active antenna a single 1N4148 can generate significant IM products. In this case it is better to use stacked silicon diodes or reverse biased silicon diodes. Another possibility is to use cheap low capacitance ESD protection diodes suitable for antennas, e. g.:
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regards
Fred
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It is a common misconception that there is a sharp forward voltage threshold for diodes and that there is no current flow below this "forward voltage". A diode is an amplitude-dependent, non-linear resistor, described by the Shockley equation. The non-linearity also exists at small voltages and is the reason why IM Products occur even with samll RF levels below the "forward voltage".
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regards,? Fred
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Icom uses a pair of 1SS302's to help protect the FE of the IC-R75 receiver.
They are very fast & I use them in a protection circuit I built for my receivers downstairs in the torture chamber (repair lab) where I torture old radios.
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-Bob
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Hi Fred,
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I agree with the points you are raising, with some caveats.
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In a 50 ohm system, a pair of 1N4148's or similar small signal silicon diodes will limit at somewhere around +10dBm, and second and third order (two tone) IMD, at average signal levels, (peaking around-30dBm max) will typically be at, or below, the noise floor of an average receiver, in an average location.
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E-Probe, high impedance, antennas are tricky, and it's usually necessary to and more diodes to a string, or provide some reverse bias, to offset the point of conduction. Loops and relatively short E-Probe type antennas generally don't develop a high enough voltage across the diodes for IMD to become noticeable. Plus, many designs are pretty poor in terms of IMD performance anyway, and I suspect that additional IMD produced by the protection diodes, would probably go unnoticed.
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Sometimes we have to trade one problem for another, and it's a user's choice which is preferable.
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Regards,
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Martin
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On Thu, Feb 27, 2025 at 11:35 AM, Fred M wrote:
It is a common misconception that there is a sharp forward voltage threshold for diodes and that there is no current flow below this "forward voltage". A diode is an amplitude-dependent, non-linear resistor, described by the Shockley equation. The non-linearity also exists at small voltages and is the reason why IM Products occur even with samll RF levels below the "forward voltage".
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Interesting question.?
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There are interesting amateur tests of the 1N4007/1N5408 as replacements for the Unitrode P-I-N characterised diodes. At least one experiment shows that 1N5408's were lower resistance than the purpose designed Unitrodes with one caveat: the recombination time of the 1N5408's/1N4007's is getting skimpy (not long enough) for perfect use at 160M.?
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or
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Considering the unknown sourcing issues of modern devices, and the clever re-marking methods, I have no confidence that parts sold with jedec registered numbers will meet specifications, much less undocumented specifications.
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Or as one person famously said, "Trust but verify!", which is why I included the links.
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Jim/VEZ
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On Wed, Feb 26, 2025 at 07:13 PM, James Redding WA9VEZ wrote:
It will be informative to follow the link below:
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Hi Jim,
Thanks for referencing my paper. :-)
However, I will not recommend that limiter configuration for protecting the Rx loop. The described Schottky assisted PIN limiter has a ~10 dB lower turn-on threshold than a conventional PIN limiter, but it trades-off linearity; i.e. the Schottky diode is capable of mixing & detecting like a small signal diode. The achieved low threshold is useful for protecting sensitive transistors, but it is not required with the regular BJTs in Rx loops.?
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Some 1N4007 have a long carrier lifetime and behave like a HF PIN Diode.
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PIN diodes are segregated into either attenuating, switching or limiting applications depending on their minority carrier lifetimes. I have seen hams using 1N4007 for attenuating and switching, but I think it is premature to decide on a suitable application without first measuring the minority carrier lifetime. Or has someone already measured them?
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73, Leong, 9M2LCL.
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On Fri, Feb 28, 2025 at 01:55 AM, <biastee@...> wrote:
I have seen hams using 1N4007 for attenuating and switching, but I think it is premature to decide on a suitable application without first measuring the minority carrier lifetime. Or has someone already measured them?
Sure, there are several measurements available in the web:
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The use of an 1N4007 as P.I.N. diode for Shortwave frequencies is well documented. See Jims link:
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https://qrp-labs.com/images/qdx/schem4.png
Adam
James Redding WA9VEZ
Bob W8RMV