Re: TX protection on RX Loop
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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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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
Hi Jim,
I wonder if a 1n4148 and a 1n4007 would work in the same way for HF?
Andrew VK5CV
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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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Re: TX protection on RX Loop
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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Re: TX protection on RX Loop
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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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Re: File upload: Rotable FLAG-antenna
Hi Fred,
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If I recall, it was an experiment I tried after reading about *tuned* spiral loops (windings in the same plane) in the early days of radio. My intented use was MW only, and I was seeking a greater amount of wire in the same space, and hoped this would work for a broadband loop also, vs. tuned. Indeed, it never worked as well as my single turn version, although it still had some good nulling ability on MW.
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Guy
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Re: File upload: Rotable FLAG-antenna
Hi Guy,
why did you choose a spiral-loop?
The enclosed loop area is a key figure of merit of a small magnetic broadband loop. With a spiral loop the enclosed loop area decreases with each winding.
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regards
Fred
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On Sun, Feb 23, 2025 at 10:01 PM, James Redding WA9VEZ wrote:
Sort of a shame the WW2 400 Hz selsyns are no longer "inexpensive" although a PM magnet, an Arduino flux compass, and some hot glue will probably replace a selsyn and all the analog hardware to remotely read out azimuth position in degrees.
I've agonised over azimuth sensing - been looking for a battery-powered device with built-in Wi-Fi or Bluetooth sender and a PC Windows receiver that "exposes" the azimuth value for easy capture by a simple program (e.g. data logger).
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Some years ago I tried a device from (via Amazon) with a display program that tries to mimic an aircraft attitude indicator. It was disorientating and made me tear my hair out. And anyway this did not let me capture the azimuth value.
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I have that exact rotator. It is fine for a 1 to 1.5m diameter loop. Although the readout only indicates 10 degree steps, it can be "jogged" to any position allowing good nulls if necessary. Tom
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There is a re-calibration sequence in the manual if it get out of whack, but I'm not sure if it covers that situation.
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Re: File upload: Rotable FLAG-antenna
Oops, this 3rd link is the one for photos of the spiral loop on rotor:
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-Guy
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Re: File upload: Rotable FLAG-antenna
I did something similar in 2008. One design was a small bidirectional loop on a rotor, and the other was a four-turn spiral loop on a rotor. Both used Wellbrook ALA100 modules. Support elements in one case was fiberglass fishing poles, and fiberglass tent poles for the other:
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73,
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Guy
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Re: File upload: Rotable FLAG-antenna
The shape isn't critical. Att that matters is loop area (length x height). Bigger area, more gain.
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/Hans
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Re: File upload: Rotable FLAG-antenna
The SULA is a smaller loop than most FLAG antennas but both bigger than a YouLoop
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Regardless they are interesting to experiment with and can use a preamplifier
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The County Wireless loop amp is a great one to use, low noise floor
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John VE3IPS?
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I have the predecessor unit and it is just a two winding capacitor (direction) induction motor with a gear train. The digital display seems to be counting AC cycles. A strong wind (or in my case power mains failure) can push a 12' yagi out of alignment. In short it is not like a CDR type with an internal wire wound resistor measuring direction.
Sort of a shame the WW2 400 Hz selsyns are no longer "inexpensive" although a PM magnet, an Arduino flux compass, and some hot glue will probably replace a selsyn and all the analog hardware to remotely read out azimuth position in degrees.
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James Redding WA9VEZ
Bob W8RMV
Adam
Guy Atkins
Hans Ostnell
John