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Newbie RF frontend protection Qs

Posted: Mon Jan 22, 2018 7:11 pm
by Strykar
Since I read a few posts about people killing their RSP2's due to overloading and a lack of RF frontend protection, I've been worried.
My RSP2 will be mounted in a remote location powered by batteries, I looked up 'RF frontend protection' and am confused what to get.
Do I want something with static discharge protection too?

Initially I was going to do: Active Mini-whip --> DC-30 Mhz lowpass filter --> Hi-Z port with a -

Then I came across a Wireless Receiver Front End Protection Unit which seems perfect, until I stumbled upon Wireless HF Antenna Isolator and Low Pass filter. And then there's even a Z Match Antenna Matching Unit Which of these three types do I want?

On the Bias Tee port, I planned on: 137 Mhz Weather Sat Dipole --> 118-138 Mhz (Airband bandpass filter) --> wideband LNA --> RSP2
Is this sensible? Is the bandpass enough, do I need/want static electricity protection here too?
I currently have no antenna attached to the third RSP2 input but I'm assuming it'd have the same 'protection' requirements as the Bias tee port.

Happy to hear suggestions.

Re: Newbie RF frontend protection Qs

Posted: Tue Jan 23, 2018 1:40 am
by Roger
A few questions.

1. Is this remote location near transmitters? If so at what distance, how much power and what frequency.
2. Do you have lightning in your area? If so you will need lightning protection for close by strikes. Nothing will protect you in the event of a direct hit.
3. How are you going to control the RSP from a remote site? It is a USB only interface so you need a computer at the remote site and that means a lot of battery power.

Re: Newbie RF frontend protection Qs

Posted: Tue May 22, 2018 4:04 pm
by glovisol
Dear Stryker,

In my opinion and according to my experience you do well to be worried about possible damage to the RSP inputs due to static discharges.

1) The Balanced input, if directly connected to a raw long wire antenna could be subjected to very high induced voltages caused by far away lightning strikes. For VLF and HF a very good earth is essential, e.g. a 2 m. length steel spike driven into the ground generally gives you a good earth. Then, whatever antenna you fancy, use a balun (Z(antenna): 50 Ohm) at the delivery end and go coaxial to the RSP. Here use another balun (50 Ohm : 300 Ohm RSP Bal input). Connect the earthing to the ground terminal of the RSP and you are done. This balun you can buy cheap enough, or make with a toroid and copper wire.

2) For VHF inputs and antennas, connect RSP Coax input (+ bias tee) to the LNA's output with 50 Ohm coax. Connect the input of the LNA to an antenna balun which will effectively provide short circuit at DC, but impedance matching at VHF. These VHF baluns you can buy or manufacture yourself with a few pieces of coax cable.



Re: Newbie RF frontend protection Qs

Posted: Tue May 22, 2018 4:50 pm
by NK7Z
Will two diodes back to back across the antenna terminals provide enough protection for large RF fields on an RSP style receiver?

Re: Newbie RF frontend protection Qs

Posted: Tue May 22, 2018 5:58 pm
by Roger
You need more than two diodes for protection if the levels are quite high. Some designs use a small incandescent bulb in series.
prot.PNG (24.68 KiB) Viewed 11454 times

Re: Newbie RF frontend protection Qs

Posted: Tue May 22, 2018 9:56 pm
by NK7Z
Thanks for the info, and schematic...

Re: Newbie RF frontend protection Qs

Posted: Tue May 22, 2018 10:05 pm
by Roger
You also might add a 10K or so resistor from the input to ground. This helps bleed off rain static.

Re: Newbie RF frontend protection Qs

Posted: Wed May 23, 2018 7:49 am
by vk7jj
Sadly, filament globes take ages to achieve maximum resistance and even when they do the resistance value is not even a tiny speed bump from the point of view of the value and time of fast rising voltages as per not so close lightning or PTT keying of transceivers into nearby antennas.

Comparing the filament's 150ms turn on time with with typical radio keying times, a quick search found: "The ARRL recommends a 5 mS rise and 5 mS fall time for CW"



Re: Newbie RF frontend protection Qs

Posted: Wed May 23, 2018 5:25 pm
by Roger
The filament bulb is there to act as a fuse to protect the diodes if the RF level is too high. When cold the bulb resistance is very low so RF signal attenuation is low. If the signal level gets too high the resistance of the bulb will increase and for very high signals will burn out and protect the diodes.

The bulb/diode design in the schematic (and variants of it) have been used successfully in contest stations for many years. You can get better front end protectors now that do not generate as much unwanted spurious due to the diodes. They use transformers that saturate at high signal levels and better diodes. For example....

You can get test reports on these from the ARRL.

This one is also popular but I have not seen a test report on it. ... n_unit.htm

Re: Newbie RF frontend protection Qs

Posted: Thu May 24, 2018 12:49 am
by vk7jj
The filament bulb is there to act as a fuse to protect the diodes if the RF level is too high.
Hi Roger, I appreciate that is the intention and I won't comment further because we're straying from the SDRPlay sandpit. I wouldn't have commented at all except for my own concern that people might embrace a solution that resulted in damage to RF front ends if they are lead to think a filament makes them bullet proof.

I use a parallel bleed resistor plus a series current limiting resistor together with a pair of high speed back-to-back clamping diodes and I'm happy with that.

And yes the RSPs already have some diode protection but I'd rather not have that be taken out by my transmissions as that would likely in practice be just as bad as taking out the entire RSP.

Neon globes are also used as a discharge path to ground but they also appear too slow.

I've also built and run with the usual RF sensing antenna-switching relay devices that one can buy but again I believe they are too slow and I do not trust them.

So, yes, the resistance of a filament does increase but the points I can't see past are 1) even when fully illuminated the resistance of a filament is still in the order of tens of ohms at best and 2) even if the resistance was meaningful which it is not, given the thousands of volts that will occur anywhere on your antenna and feedline at any of the many high impedance points that are always present at any particular frequency you might be transmitting on or receiving induced voltages on, 3) the speed of the increase is so slow it provides no protection at all against the things we most need to be protected from, ie the keying of a nearby transmitter or possibly some weather related lightning event.

Regards, Phil