Batboard

In the 15 years I have been installing repeaters I have been installing a quarter wave shorting stub at the entry of the site building which is tied directly to the grounding cable that leads to the outside 8' ground rod. I make my own of good quality copper and brass. The stub not only appears as a direct DC short to ground but offers varying attenuation to all other frequencies to ground except at a quarter wave of the operating frequency. It provides lightning protection as well but I alway add a Polyphaser also. The transmission line does not see this short on the line at the operating frequency because of zero RF current at the quarter wave and odd multiples The VSWR meter does not see it either. It's somewhat broadbanded. For example at 450-470 MHZ the same stub will work over the whole band. At the low and high end the VSWR wil indicate a slight mismatch, so the filter can be made slightly shorter or longer depending where in the band you operate. Any comments about this feature or use of it.

[quote="windmillsforsale"]In the 15 years I have been installing repeaters I have been installing a quarter wave shorting stub at the entry of the site building which is tied directly to the grounding cable that leads to the outside 8' ground rod. Any comments about this feature or use of it.[/quote]

What your doing is what everybody should be doing at their radio sites. Problem is that not many of the so called "engineers" have been involved with this type of protection. Their instructors in school never told them about this method of protection. SO AS A RESULT, it doesn't get installed or talked about anywhere.

The 1/4 wave short has been around since almost the begining of radio. The main reason it doesn't get used much is that most people don't know how to calculate the length of the shorted coax cable. They get scared and stick with the proven "Polyphaser" or other gas tube method.

"Andrew Corp" makes the shorted stub protectors for 800 and the 1900 Mhz frequencies. I haven't seen that many companies using them though. Again being involved in the cellular market for over 14 years, I have asked about this type protector. Most of the time I get the standard answer "that is what the company uses and not going to chage now" from the company RF engineer. Also bear in mind that most of these engineers are only a couple of years out of school. Most of them are still wet behind the ears.

Why would they listen to someone that has been in the radio field for better than 35 years. Your a threat to their job security. Their way has worked so far and they don't want to chance making changes that could possible go sour on them.

That's why there are field techs running around the cellular systems after a bad thunderstorm. I love this job secrity way of doing things. Only problem is the stock holders keep asking why so much money is being spent on lightning damage repair.

Last part of this topic is the grounding system. If your ground is not good and installed properly, it doesn't matter what type of protection you put on the coax. The site grounding system makes or breaks a radio site. Many sites have a poor ground "system" and there are constant repairs being done.

One of the first things I do when called to look at a site is pull a shovel out of the truck. Then start digging at the ground wire going into a building. It's amazing the things that get covered up underground. Most of the time it's a poor earth counterpoise system if there even is one.

I have seen sites that just had the # 2 wire in the ground about a foot. Ran it about 6 or 8 feet away from the building and made a couple of loops so it wouldn't pull out if you yanked on it. For this they got paid for installing a loop around the building, a loop around the tower and ground rods every 16 feet along the counterpoise. The exothermic welding that was to be done to the ground rods that were not there sure made a good deposit in the bank. The tower ground was not even connected to the building except through the coax.

These are the type of installations that receive lightning damage. The contractor walks away big fat and happy. The radio carrier can't figure out why he keeps getting blasted every time a storm goes through.

It all comes down to watching over the sholders of the grounding contractor. I always demand to see the underground wire connections and welds. If they cover it up before I get there, the contractor will spend the next day digging it up for inspection. It's part of the contract to have the grounding system inspected before it is covered up.

I have kind of got off the main topic, but it all goes hand in hand. Hope this gives you some ideas.

Jim

Jim, I don't think this is off topic as it is one of the most important parts of infrastructure. Very good post, thanks.

Also, on the quarterwave stub, another important part of the infrastructure. Thanks wmfs.

We used 1/4 wave stubs and star gaps on the RF and carbon gaps on the telco lines way long before the gas tubes became available.

OK, just to make sure I understand the post, you are talking about a "quarterwave SHORTED stub". Not a "shorting stub".
I have been working on and at some new sites, where the grounding is absolutley FIRST RATE! Something that is very rare in my experience.
But I have not seen the shorted stub you are describing.
Can you please expand on the description of how you make and install them? In a PM ore direct email if you prefer.
I have recently moved from Washington state (where lightning is less a problem) to Nevada (where its a BIG problem).
One of the sites I am helping to rebuild is in dire need of protection, and stubs would be an improvement, at small cost.
I have used stubs for passing or notching, but not lightning.

The detail on this would be great posted right here. I am very curious about this too.

"Shorting stub" is the correct word. In the vhf and uhf world most transmission lines coming down the tower are either 1/2" or 7/8" hardline and even bigger, coaxial cable. RG/8U was used for shorter runs. The coax is terminated into an N or UHF type connector where it enters the building. A coaxial tee is installed on the connector and the coax running over to the repeater or base comes out of the top of the tee and the shorting stub fastens to the bottom of the tee. The shorting stub can be a piece of coax a quarter wave length long and at the end the center conductor and the shield are fastened together and then run straight down to your grounding system.

After some expermintation, using formulas etc. I came up using 1/2" type M straight copper tubing, a copper end cap, a 1/4" brass rod, a piece of .010 brass shimstock 3/8"x2" and a N connector RFN 1002-1N. For a stub to work with 450-470- MHZ. cut the 1/2" copper tubing 5-1/8" long. Open the connector bag and take out the outer connector shell and the pin. Discard the rest. Wrap the shimstock inside the back of the connector and insert one end of the copper tube. Place the connector face down on the bench and insert a screw driver into the top end and with a small hammer tap the top of the screwer to force the tubing all the way into and bottom out into the connector. About 3/8" . Take the end cap and drill a 1/4" hole in the exact center. Machine one end of the brass rod down to .100, 3/16" long. Solder the pin on this end. Tin the 1/2" tubing where the cap will slide over. Now insert the brass rod with the pin through the end cap down into the connector. Some resistance will be felt seating the pin. This leaves 3" sticking out to clamp your ground wire to. Use a propane torch and solder the end cap at the joint and where the brass rod protrudes through the cap. DO NOT have the connector face down when soldering as solder may run down the brass rod inside and short out at the pin. Keep it level and rotate the stub when soldering. Use just enough heat to melt the solder then get off.

These dimensions of 1/2" copper and 1/4" brass rod will come out very close to 50 ohm impedance. I stay wilth type N connectors. If you build this and use an UHF adapter then you must make the stub shorter to compensate for the extra length. If you need one for 150-170-MHZ the stub will be approx. 3 times longer. When checking VSWR. Place a tee on the antenna side of your wattmeter. A 50 ohm load connected to one side of the tee and the stub to the other. As mentioned in my first post I also install a polyphaser between the tee and the coax going over to the equipment. I would send a photo but don't know if this forum supports photos.


I hope this explains more and answers your question.

Dave..

Yes, I understand.
I have seen a few made from LDF-4, where they just used the LDF in place of the copper pipe.
And soldered copper grounding strap accross the end, with the hole drilled thru for the center conductor.
It would work the same as long as one takes the velocity factor into consideration and adjusts for the length of the connector.
Now I understand what they were doing.
I wasn't sure if they were just using the shield on the LDF, and not the center conductor. Just using it instead of the grounding kit.
Very good ideas.
Makes me think....
Thanks for the description.
I will build a couple, and use the tracking gen to find the right length.
I would assume the same will work for VHF Lo, has anybody had any experience with that?

I should have mentioned that the 1/4" brass rod is 9" long for the above instructions to make one for 450-470 MHZ.

I have seen shorting stubs that were apparently factory installed in some low band Mastr 2 stations. I once got a going over by an ill-informed technician for putting one back on.

Dave...
Thanks for the details on this! I remember seeing this info in an old GE Datasheet that was in with some old LBI files I picked up. I am involved in a site upgrade project and these stubs will be used. Simple, cheap, and effective...

Josh

I use one to keep the 25kw FM station out of my 6m repeater.
Without it I can measure several watts of 100.5 country music coming down my feedline.


Chris
N9LLO

I've been reading this with some interest - I always used a polyphaser with a solid ground, but this sounds like an elegant addition.

My question is would maching a mating "N" out of solid brass work any better? IE it would be a solid short, with no insulator? Or would the tiny center pin negate any advantage?

The other end of the "connector" would be machined to be exothermic welded to say a 2/0 grounding wire leading to the mat/radial system.

Martin

I went down to the shop and made several of these stubs today. Got all the necessary parts except for the N connectors at the ACE Hardware around the corner. We had a bucnh of old N connectors that had ben cut off of cables that we cleaned up and used. Knew that old junk would come in handy. I checked them out using our SiteMaster analyzer. The UHF stubs seemed to be more wideband than the VHF. I'll try making some more soon, for 900MHz.

Any ideas for spacing the center conductor when making them for low band??? There are Nylon bushings at the hardware store. Anyone think that would be OK to use??? Even on the VHF ones it would be nice to better support the brass rod at the connector end.

Once you get the methods down, these are real quick and easy to make. I think I could now make about 8 to 10 per hour, including the time to check each on the analyzer. Seems like a very durable and cost efective solution.

Any idea what power levels these home-made stubs will handle???

Josh

P.S. It's good to see an build it yourself thread here. Nothing beats a fun, easy project that is effective and a money saver. I really like the idea of a real, solid ground on my feedlines.

I went down to the shop and made several of these stubs today. Got all the necessary parts except for the N connectors at the ACE Hardware around the corner. We had a bucnh of old N connectors that had ben cut off of cables that we cleaned up and used. Knew that old junk would come in handy. I checked them out using our SiteMaster analyzer. The UHF stubs seemed to be more wideband than the VHF. I'll try making some more soon, for 900MHz.

Any ideas for spacing the center conductor when making them for low band??? There are Nylon bushings at the hardware store. Anyone think that would be OK to use??? Even on the VHF ones it would be nice to better support the brass rod at the connector end.

Once you get the methods down, these are real quick and easy to make. I think I could now make about 8 to 10 per hour, including the time to check each on the analyzer. Seems like a very durable and cost efective solution.

Any idea what power levels these home-made stubs will handle???

Josh

P.S. It's good to see an build it yourself thread here. Nothing beats a fun, easy project that is effective and a money saver. I really like the idea of a real, solid ground on my feedlines.

[quote="kd6kml"] Any idea what power levels these home-made stubs will handle???

Josh [/quote]

Josh, Don't think power rating is the issue here. Your not trying to pass power through it, just take any voltage to ground. If they are built to the right length, they will never see any power.

Another point to ponder here is an application that hasn't been brought up yet. That is rain and snow static. Many low band antennas are not DC grounded. They are very prone to static build up on the antenna. This voltage will build up till it arcs over someplace. Many times it is in the receiver front end. The old time radio techs use to put neon lamps across the receive antenna connection to try and keep this voltage down to an acceptable level.

Another way to reduce the voltage is to put in some hot carrier diodes back to back on the FET side of a coupling cap or resistor in the antenna RX front end. The use of a 1/4 wave shorting stub would eliminate any static build up on the coax or receiver input. Would work much better than the combined neon lamp and hot carrier diodes. Wouldn't have to worry about it going bad after a lighning strike.

Just remember that the best lightning protection is only as good as the ground it is tied to. Poor ground system and you will never have your equipment survive. I don't care how many protective devices you put on the antenna, power or telephone lines. The bottom line is poor ground, poor protection. No one thing by itself will make the protection work. It takes a system of several parts to work together to have good surge protection.

Jim

Machining a brass connector to provide a short at the N connector would defeat the whole purpose of what we are wanting to accomplish. In theory the RF current traveling down a coax line is zero at a quarter wavelength of the operating frequency. It is also zero at 3/4, and 1-1/4 wavelength and so on down the line. When shorting the center conductor to the outer jacket at the point of no RF current, nothing happens. The transmitter, receiver, duplexer, VSWR meter, or what ever is next to it won't see it, assuming they all have 50 ohm terminations. It appears as an open, but DC wise it is a direct short. All other frequencies are attenuated in some degree because they see an impedance to ground.

Long stubs would be suited for a spacer down halfway to the pin to provide some tension relief. Teflon would be good.

As Jim pointed out static voltage build up is bled off do to the stub.

Imagine a tower is hit by a 10,000 amp lightning strike maybe for 3 seconds. Assume the tower takes 90% of the energy to ground. Assume 10% races down the outerjacket of your coax line to ground. That's 1,000 amps. Can you imagine how much of a magnetic field that is created by 1,000 amps?
The magnetic field and the colapse of it is induced into the center conductor by transformer action. A very high voltage spike can result. Another reason for the stub to provide a path to ground

When I made mine I used just plain rosin solder. Silver solder would be better. I didn't solder the connector, brass shim stock or copper tube. I didn't want that much heat near the center pin and insulator The friction of the press fit should suffice. Heat shrink could be used and of course if used outside should be weatherproofed.

Windmill

Check your private mail.

Jim

windmillsforsale wrote:Machining a brass connector to provide a short at the N connector would defeat the whole purpose of what we are wanting to accomplish.

Actually what I was asking about was using a shorting connector *after* the tuned stub.
Sideport on "T" -->Tuned Stub --> Shorting Connector --> Really good earth ground.

Sorry about the confusion!

Martin

I've been following this with some interest. Before I encountered some health problems, I did some site installation for a Motorola shop, including the feedline grounding kits, Polyphasors, etc.

My question about the use of a shorted stub as this:

What about the low temp solder? In the case of a pretty much direct hit, seems to me the low temp solder would be worthless. Seems to me that if you would make the stub out of a good, heavy copper tube, say some scrap "big" hardline, or calculate the diameters, and obtain some hard copper from, say, a refrigeration outfit, then use something like "Silfoss"--the high temp silver braze material we used on refrigeration tubes.

Or maybe, the low temp, you think, is good enough? Maybe in the case of a hit hard enough to melt the solder, the radio is "toast" anyhow?

Delar, W7DJM

To answer the two questions. The use of a shorting N connector at the end of the stub would work OK but then it's another connection, another place for arcing to occur. Hence the reason for the 1/4" brass rod.

About using low temp solder. Your right and I considered that to maybe be the weak link to my design. That's why I added a post about using silver solder. During a strong hit would the pin vaporize or the solder melt? Actually, brazing would be best.

Dave..

Good points about the stubb. A side benefit is a bit of out-of-band attenuation to help keep cross-band IMD in check.

Don't forget that if you take a hit, and your stub does successfully protect your radio front end from differential voltages, your whole site will now be "floating" at many thousands of volts. The Ground Potential Rise will want to exit somewhere. Like the 120/240VAC or telco copper tethers. And thus you must also provide your preferred path for the stuff to leave. A nice surge device on your mains will keep your power supplies from providing that service.

RFDude

[quote="RFdude"]Good points about the stubb. A side benefit is a bit of out-of-band attenuation to help keep cross-band IMD in check.

Don't forget that if you take a hit, and your stub does successfully protect your radio front end from differential voltages, your whole site will now be "floating" at many thousands of volts. The Ground Potential Rise will want to exit somewhere. Like the 120/240VAC or telco copper tethers. And thus you must also provide your preferred path for the stuff to leave. A nice surge device on your mains will keep your power supplies from providing that service.

RFDude[/quote]

If the site does take a hit, it is hoped that there is a "good working system ground" there. No one device regardless of where it is installed will protect a site. The protection is a "SYSTEM' that encompases many different devices and methods. You could spend days talking about it.

Bottom line is a site needs a good earth ground "COUNTERPOISE" with the ground rods attached. You need surge protection on both the AC power and the Telco lines. The radio cabinets need to be grounded to the common "counterpoise ground ring. The antenna coax cables need to have multiple ground kits on them. Many services use a common ground bar in the shelter called a "Master Ground Bar" as a common point to tie everything to.

Take any one item except for the "counterpoise away and you may or may not receive damage from a strike. At least the chance of survival is greater when all the inividual pieces are there and connected.

Jim