Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Gas Discharge Tubes (GDT) Long post, sorry  (Read 606 times)
N4UE
Member

Posts: 927




Ignore
« on: May 13, 2019, 03:27:32 PM »

Hi. As my interests in this wonderful hobby ebb and flow, I've recently become interested in GDTs.
I have a bunch of MFJ 1704 switches and recently acquired an Alpha Delta 4 position switch.

I found there is little information from either company about testing/replacing these devices.
Google being "my friend", I found more information about these things, than I will ever be able to digest.
I just wanted to understand the failure modes and how to test.

OK, some VERY detailed testing from the Mfg. revealed some aspects, even they couldn't understand. Anyway, the 'failure mode' is described as a short from repeated arcs depositing material on the inside of the ceramic 'tube'. OK, I understand.

The Alpha Delta has a very easy to access element. My switch had a big ole Allen screw REALLY tightened down to the point it was bending the back cover. I've since ordered several replacement elements with the correct plastic thumbscrew/o-ring.

I have several ohm meters that can measure many megaohms, and the tube is ok.

Next the MFJ 1704. There is NOTHING I could find about replacing the 'replaceable element'. So, I pulled the back off of one of my spares.
You can't check the resistance due to the way it's mounted. (always grounded). So, I unsolder the one end going to the ground 'clip'. To my surprise (??) the GDT was not connected to the other (common) end. It was soldered at one time, but it was now floating in a bunch of hot glue.

I took it out, it tested OK, I cleaned and re-soldered both ends where they should have been.


One final thought..... on the MFJ, the 'common' brass strip that one end of the GDT was attached to was only about .005" (or less) from the casting. It was just way too long, but impossible to shorten where it is, without a lot of disassembly..
In addition, the metal strips that connect from the 4 coax connectors to the 'common' board are VERY, VERY close to the casting. It appears they were soldered with no regard to placement.  Huh
These are supposed to be good to 500 MHz.
In the near future, I'm going to put these (and the AD) on my SA/TG and see how they perform.

In all honesty, in my particular installation, I prefer the MFJ: '1 at the bottom, 4 at the top' physical layout. MFJ has a nice casting. Everything else is pretty poor.

I've done a couple of 'reviews' of these MFJ switches here. Some good, some not...…. Semi-kits.

I've enjoyed learning about GDT, since I live near the "lightning capitol of the USA".... ha  ha

ron
N4UE
Logged

If you're not the lead sled dog, the view never changes......
K6AER
Member

Posts: 5745




Ignore
« Reply #1 on: May 17, 2019, 02:59:54 PM »

Ron,

Sorry no one has replied to your inquiry.

Gas Discharge Tubes are nothing more than two electrodes with a conductive gas encapsulated, generally, in a small glass module.  There main purpose is to save the switch contacts.

Several things happen when they see a high enough voltage to start conduction. The first is the threshold voltage which can be anywhere from 300 volts to 1000 volts depending on the specific part number. The arc over will take generally from 8-20 uS to conduct. When conduction takes place the arc voltage can drop to a low as 15 volts across the terminals until the voltage surge has dissipated. Now note that until the tube arcs, the voltage can get past the tube and do much damage down stream from the protection device. When the voltage dissipates below the sustainable ionized conduction, the Arc will stop. If the voltage has high enough current to it, the GTD can explode from heat.  When looking at the GDT if the glass is black it has taken a hit

Other surge devices such a MOV’s and surge diodes are very fast but handle limited current and cannot be used in a RF application due to their capacitance. GDT have to be rated in antenna switch application according to the power they will see under normal use. A 300 volt GDT is fine for a 100 watt transmitter but a KW amplifier should us a 1000 volt GDT in the switch.

No one device will save the station. Surge protection is a whole house engineering solution that works most of the time. With out a excellent ground the surge protectors have no path to dissipate lightning energy.
« Last Edit: May 17, 2019, 03:06:18 PM by K6AER » Logged
WA7ARK
Member

Posts: 755




Ignore
« Reply #2 on: May 18, 2019, 06:43:29 AM »

... With out a excellent ground the surge protectors have no path to dissipate lightning energy.
I think that statement needs explanation.

If lightning strikes close by, there are transient currents induced in all conductors.  Most of that current occurs because those conductors are connected to earth in more than one place, and there is a voltage gradient in the earth as the lightning energy is dissipated. Inter-Connecting multiple grounds (like the ground rods under a tower, the ground rod at your station entry panel) to the electrical service ground using #6AWG external-to-the-house bonding wires is an effort to control the peak voltage between all of those earth connections. It also minimizes the peak currents that might otherwise flow along the in-wall house wiring (between outlets and the electrical distribution panel). About 95% of what you can do to protect ham equipment comes from this method.

In this context, the only thing that GDTs can add to the above is to clamp the peak transient voltage between the center conductor and the shield on a given coax run. The current path is through the GDT itself. The GDT has to dissipate the energy in the transient inside the capsule.








Logged

Mike, WA7ARK
N4UE
Member

Posts: 927




Ignore
« Reply #3 on: May 18, 2019, 12:09:25 PM »

Both Mikes, thank you for your replies.

Although I've been a ham forever, I enjoy learning about subjects I haven't dealt with before.

Having acquired both the MFJ and Alpha Delta (4 position) switches, I could find little information from either Mfg concerning testing/replacing the GDT. AD did have some links that were helpful.

Once the GDT does it's job, how can you tell it's "charred", since it's ceramic?
I assume you would use a high range ohmmeter to check for leakage.
Or would a better approach be to just replace the GDT every year?
I live close to the lightning capitol of the USA, so I like to be prepared. ha ha

thanks again!

ron
N4UE

PS: K6AER, the 'hobby' farm of a buddy I work part time at, we have 2 Border Collies. Every year, rated the "Smartest Dog". No exaggeration, they are amazing. They LIVE to work and herd sheep and cows so well, it's awesome!
Logged

If you're not the lead sled dog, the view never changes......
K6AER
Member

Posts: 5745




Ignore
« Reply #4 on: May 18, 2019, 03:40:17 PM »

"In this context, the only thing that GDTs can add to the above is to clamp the peak transient voltage between the center conductor and the shield on a given coax run. The current path is through the GDT itself. The GDT has to dissipate the energy in the transient inside the capsule. "

With out a low impedance connection to Earth (Ground Rods) the ability of the GDT to clamp voltage and pass current will be compromised. GDT do not dissipate energy, it connects the energy to the grounding system.. Only earth dissipates the energy. Using a large conductor copper wire for ground connection is wishful exercise if the connection to earth is poor. Typically I like to see ground rods have under 2 ohms of earth resistance as measured with a AEMC meter.

As I have said before, 90% of lighning damage comes in via the AC panel from the power company. If you don't have a surge protector on your AC panel your radios will become fuses when lightniing surge is headed to the best grounding system in the area...your antenna system.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!