Re: Topband: Receiver protectors.
An important point worth mentioning here is that the life of gas discharge tubes is reduced every time they conduct. Exactly how much depends on the arc current, which is explained in the Bournes GDT data sheets, but their life can be significantly extended by the following simple and inexpensive modification. After I read that data sheet, I started paralleling 33k Ohmite OX resistors across the 90v GDTs (between each Beverage wire and ground) to prolong their life. Those resistors provide a DC path to ground, which constantly drains static charges to ground. This eliminates many small discharges through them from the inevitable static charges that would otherwise build up, thus prolonging their useful life. These Beverages have a surge impedance of about 450 ohms. Therefore, 33k (some use 47k, 10 times the surge Z) resistors will have a minimal effect on received signal strength, RDF, or the required matching transformer impedance ratio. For a higher impedance antenna, the drain resistors should have a correspondingly higher value (and perhaps a TVS diode should also be in parallel with the GDTs and resistors?). Some well-meaning hams have recommended much higher value resistors as a static drain, even at a transmitting antenna feedpoint where GDTs are seldom practical. However, if the resistance value chosen is as low as practical, then the GDT's life will be maximized. 73, Mike W0BTU _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Receiver protectors.
Jim, Good data. It looks like the RG-5000 consists of two back-to-back diodes (in IC1) with the other components not contributing. Do you have a part number for IC1? GDT/ES1A diodes The GDT (gas discharge tube) doesn't appear to help. By the time it conducts at 90 volts the diode string current is 160 amps and the diodes will have failed. During a fast rising pulse (<1us) the GDT voltage can rise to 1000 volts before conducting and the diodes will fail from excessive current. When the GDT does conduct it may clamp around 15 volts. ES1A/IC1 IC1 appears to contain two back-to-back diodes. The output limiting voltage is that of a single diode, preventing the ES1A diode string from conducting. Fed by a 10 watt, 50 ohm source the average current thru IC1 is 900 mA. Multistage clamping protector circuits require limiting impedance between each stage. For the RG-5000 circuit the diodes do all the clamping until the GDT conducts. For the case where the GDT takes 1 us to conduct with a 1000 volt pulse (lightning) the current thru a diode should be limited to 10 amps. Small signal diodes I've used in clamps survive 10 amps for 1 us. The limiting impedance between the GDT and the diode clamp needs to be 100 ohms. As 100 ohms impacts insertion loss the diode clamp requires parallel diodes so that a smaller resistor value can be used. If we make the resistor between the GDT and the diode clamp 10 ohms we need to parallel 10 diode strings. The capacitance will be somewhere around 20 pF which is okay past 30 MHz. A limiting impedance must also be placed between the ES1A string and IC1. Instead of this I would simply use 20 ES1A diodes with 10 paralleled to clamp negative and 10 paralleled to clamp positive. The circuit is then: GDT -->10 ohms --> diode clamp The resistor must take 1000 volts for 1 us and an Ohmite OX series resistor will do. If a clamp voltage greater than one diode is desired I use steering two diodes to a zener diode clamp instead of series-connected diodes. The zener/capacitor can be biased with a small DC current so that the RF signal doesn't have to charge the zener/cap. QST RG-5000 review https://static.dxengineering.com/global/images/technicalarticles/dxe-rg-5000_sn.pdf Dave KH6AQ On Tue, Mar 17, 2020 at 5:27 PM wrote: > In a thread back in January, 2020 on connecting receive antennas for top > band, a few fellows mentioned a review that I wrote in 2014, where I > compared the DEO RG‐2000M and DXE RG‐5000 Receiver Protectors. > > > > I have updated my review with new measurements, additional theory of > operation, more details and some bug fixes. If anyone is interested, it is > at https://www.w8wts.radio/RXProtectorsREV03-2020.pdf. I hope that people > find it interesting. > > > > Be safe everyone - stay home and get on the air. > > > > 73, > > > > Jim, W8WTS > > > > _ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Topband: Receiver protectors.
Last year I purchased an Airspy HF+ Discovery SD receiver. Eventually I got around to trying to use it with a second hand Windows laptop. I was concerned about blowing out the front end on transmit because I had heard of that happening with these receivers that are not designed exclusively for ham radio. A lot of the SD receivers are designed and manufactured by non-hams for lots of uses other than ham radio. They are not made to be used in an environment where they are in the near field of a transmitting antenna excited by a few hundred watts or more. I looked at the DX Engineering product and saw this specification: Max Output Level: RG-5000HD:+14 dBm at 10 W input. RG-5000: +10 dBm at 10 W input The corresponding specification for the Airspy rx is: +10 dBm Maximum RF input These are sensitive receivers. The DX engineering products were marginal so I did my own thing. I put the rx in an aluminum box with a UHF jack mounted on it an a grommetted hole for the USB cable's exit. I mounted a pair of relays in the feedline to the receiver inside the box and put RF chokes in series with the DC line to the relay coils (24 v. DC) at the entrance. I figured two relays in series in the line would add some protection. Amazingly none of this helped much as a 20 w. carrier on 160 m. produced a disturbingly strong signal trace on the receivers panadaptor. I wanted to see little or no signal at a few hundred watts to feel comfortable about operating at higher power. I was using typical ice cube style relays. The contact spacing isn't much so I think the relays were just acting like low value air dielectric capacitors in the line. RF went right through them. What finally did the trick were a pair of small DowKey style coaxial feedline relays with SMA jacks on them. Something kind of like these: https://www.ebay.com/b/SMA-General-Purpose-Relays/36328/bn_7684024 I found them at a hamfest and bought two because they had 12 v. coils so I figured I could put two in series with their coils in series for my 24 v. keying system. The same hamfest also had a guy selling small coax jumpers with SMA males on them. Perfect. I wired them up and put them in the aluminum box with the rx and all the other stuff. That did the trick. I see little or no signal on transmit and the SDR is safe at high power. I really didn't want to zorch a $170 throwaway item by making an avoidable mistake. It took a long time of fiddling and experimenting to get it right so I'm posting these notes to save others some time. 73 Rob K5UJ _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Topband: Receiver protectors.
In a thread back in January, 2020 on connecting receive antennas for top band, a few fellows mentioned a review that I wrote in 2014, where I compared the DEO RG‐2000M and DXE RG‐5000 Receiver Protectors. I have updated my review with new measurements, additional theory of operation, more details and some bug fixes. If anyone is interested, it is at https://www.w8wts.radio/RXProtectorsREV03-2020.pdf. I hope that people find it interesting. Be safe everyone - stay home and get on the air. 73, Jim, W8WTS _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
