Protection for RF High Power -

Hi iam looking for a protection solution for RF on my Station
RF Vox maybe to slow ?
Any idea?

73 55 de DL5QY


  • Hi,

    I use a pair of back to back IN4148 diodes connected in parallel with the RF input to my KiWi SDR.

    This limits the RF input to about +10dBm which mode receivers can withstand.

    As long as the diodes are connected across a low impedance termination (50R) they will not significantly contribute unwanted intermodulation products.

    If you want something a bit more sophisticated for a lot more money:-

    DX Engineering have the Receiver Guard Electronic RF Limiters DXE-RG-5000 for $79 USD

    Array Solutions have the AS-RXFEP - Receiver Front End Protector

    These consist of a pair of Mini-Circuits broadband transformers and a couple of limiter diodes, but they charge $69 USD

    You could build one yourself using the circuit on the above webpage for about 1/3 the cost.


    Martin - G8JNJ
  • @G8JNJ Hi Martin, I'm curious how this has worked out for you? I had previously put a TVS on mine but it's capacitance was creating a low-pass filter. Anything above 40m was severely attenuated! The capacitance of the 1N4148s is much lower (4pF) and that's what I'm using now and it makes a huge difference. I'd like to know if you feel the protection is working. Also has any lightning or local transmitting damaged the diodes? From the spec sheet and my math they should be able to handle about 10V and 200mA or so before they zorch. I'm considering adding a small series R (like 5-ohms) before the diodes to make them even more effective without significantly affecting performance.

    Nick W1NJC
  • Do the 'normal' TVS parts not work, e.g. SP4203 ?
  • edited September 2019
    Hi Nick,

    I'm using several types of active and passive antennas and I occasionally TX using a wire antenna that runs within a few metres of the antennas at power levels of up to 250w.

    I have 1N4148 diodes on all my RX inputs (including the KiWi's and I don't bother disconnecting the when I TX. So I'd say they work OK for me, however depending upon you exact installation "your mileage may vary".

    However there are better low capacitance alternatives that would probably catch really fast EMP type lightning pulses like the SP4203 that Glen has mentioned, so I may well test those sometime in the future using my homebrew ESD gun, based around a modified 'Poundshop' (Dollar store) BBQ Piezo Gas Lighter :-)

    EDIT - I just looked at the SP4203 datasheet and I note that the breakdown voltage is about 6V and the clamping voltage is around 10V, so although OK for ESD protection, they don't look suitable for RX input protection against excessive RF levels.

    Rather than use an input resistor I think I'd take a look at using the small 1:1 transformers in the Array Soultions and DXE products which saturate at high levels of RF.

    You could always use a combination of 1N4148 and SP4203 for a 'belt and braces' solution.


    Martin - G8JNJ
  • edited September 2019
    I just looked at an in-circuit SP4203. Driven from 1-30 MHz from a 50 ohm source and terminated with 50 ohms it nicely catches peaks greater than 7V. Actually it catches at 7V and then drops back to ~6 V peak at the top/bottom of a sine wave. Admittedly this is still not a small signal.

    I too don't know if this will be adequate protection. My main concern at the moment is to protect the inputs of a push-pull/parallel GaLi-74+ LNA from lightning damage. Colorado, where I live, has one of the highest incidences of thunder&lightning in the US and I've already destroyed about a dozen parts from nearby lightning strikes. A cloud-ground strike <500m distant (which I personally directly saw ) took out half the devices in one preamp that was connected to a half-wave 15m vertical dipole while a similar preamp driven by a horziontally polarized dipole survived. However during the course of a different storm all devices in both preamps were lost.

    Some components, such as the GaLi-74+, claim special transient-protection robustness but I think that low IMD, high bandwidth parts in general tend to be more fragile. I don't know whether the SP4203 will be sufficient to protect these MMICs from the next storm or not. I've added them to the repaired preamps. Doing so seems to have made no measurable difference in the amplifier performance, either gain or IMD. Whether these parts can withstand 14V pp may be a very good question and perhaps a lower voltage clamp, one that risks introducing IMD, may be necessary. It's worth noting that the KiwiSDR has already survived lightning storms while driven from the same dipoles but with 3.3V clamping TVS. Each storm is different so this is not a definitive result but perhaps the SP4203 is simply the wrong choice while a <a href="">different part? may suffice.

    I suppose in the cause of science I will now see if the preamp that includes the SP4203 can now survive Colorado storms or not.

    I'll try to report any results.

    Glenn n6gn
  • Hi Glen,

    The GALI-74's and PGA-103's do not like it up them, and I've just recovered a GALI push pull pair in a loop amplifier that has blown up for the second time this week, and there's no lightning about either.

    The problem with nearly all modern small signal RF devices is that they are very susceptible to damage from ESD or any other short term RF overload. This is mainly because the device junction materials are so thin they can easily be punctured.

    I've played with some very low noise 2.8v transistors that that have an Ft of 45GHz, but I managed to get through 20 of them in less than an hour, by just testing a circuit on the ESD protected bench.

    It's becoming increasingly difficult to obtain many of the old favourite ~1w through hole VHF power transistors that are commonly used in HF pre-amplifiers and active antennas. Many of the near equivalent surface mount devices have much higher values of Ft, which make them great to use, but very very delicate for the type of use we would like to put them to.

    I recently designed a commercial active antenna using such devices, but the final circuit ended up with so many protection components you wouldn't believe it. As you stated it's also difficult to add protection without unduly degrading the basic RF performance of the circuit you are trying to protect.

    If it was easy, everyone would be doing it and mil spec would be less costly :-)


    Martin - G8JNJ
  • Thanks for the responses guys. I have some SMP1330 PIN diodes coming that I'll try out too. I believe the 1N4148s will be fine. At the moment I actually have 2x 1N4148 and 2x 1N5817 on the high-z (beverage antenna) side of my 9:1 transformer going to the kiwi. The capacitance might still be having an effect but it's way better than it was before. The TVS I was using was P6KE6.8CA. I think maybe the SP4203 in addition to at least the 1N4148s might be nice to supplement ESD protection. The 1:1 transformer that saturates is a good idea too. I do not know the saturation point of my 9:1. It's hand-wound (2 windings - voltage configuration) on a small binocular core.

    Nick W1NJC
  • Some additional notes on LZ1AQ's pages


    Martin - G8JNJ
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