Balun for Sector

Hey guys,

Was wondering what is the smallest coax someone can use to make a Balun for this Sector antenna.

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is the RG-58 is so short and hard to bend' Can you use a smaller diameter coax and still have this antena work? Thanks guys Pete

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Read the article again. The author uses RG-316/u teflon dielectric cable for the balun. Outside diameter = 0.100". The RG-58a/u is used for the feed line (which is pushed through a brass tube and soldered to the PCB reflector). For such short runs, the coax cable losses are not important.

I use semi-rigid 0.087" diameter coax because it's easy to handle, easy to solder, no messy braid, and I happen to have some. There are smaller sizes, but those are far more difficult to handle:

If you can find one, get a thermal wire stripper for teflon. If not, use a sharp Exacto knife, or make your own hot wire thermal stripper.

There may be some differences in the strip length of the balun coax. Length = 0.5 wavelength * velocity_factor Most of the stuff I use has a measured Vf of 0.70, but there are some oddities floating around, especially if you buy surplus. Measuring the Vf might also be helpful if you can't identify the cable.

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Jeff Liebermann

Time Microwave LMR-100

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Jeff not sure what RG-316 is, but is it small like LMR-100 like DTC says?

Now that would be nice if I can that! What would be the length to use if in fact I can use LMR-100?

Thanks guys Pete

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Both LMR-100 and RG-316/u are about 0.100" in diameter. LMR-100 is stiffer and does not bend easily. RG-316/u does not have a foil shield, and therefore bends much easier. Either will suffice.

See photo of RG-316/u on bottom of page.

A suitable alternative would be RG-188/u which is 0.110" diameter but has a tape wound PTFE outer jacked instead of FEP.

The velocity factor of LMR-100 is 0.67. One wavelength at 2.44 GHz (mid band) is 12.3 cm in free space. The shield strip length (not the cut length) is: length = 0.67 * 12.3cm / 2 = 4.12cm See the diagram for how the balun is built.

Incidentally, I've now built about 6 assorted AMOS/Franklin antennas including some really weird constructs. They all work just fine, although the higher gain versions required using a sweep generator to accurately adjust the center frequency.

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Jeff Liebermann

Excellent news Jeff! Thank you. If you look at my sector here

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'll notice that the copper tubing is soldered to the "N" female connector underneith and also to the main board.

I saw diagrams how to solder the shielding together at the Balun but since I'm soldered to the connector AND the backplane then would it be safe to say that I can then solder the Balun shielding straight to the copper tubing?

I also changed the design of the biquad. What do you think now?

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I really appreciate and respect your input sir' Thank You Pete

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More links on AMOS and Franklin antennas from my messy bookmark collection:

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You'll notice that the copper tubing is soldered to the "N" female

Looks about right. What type of coax is that shoved into the copper tube? It kinda looks like aluminum foil outer shield, which won't work because it can't be soldered. You don't really need the shield on the coax as the copper tube provides that function.

Also, that's a Radio Shock soldering iron which is grossly underpowered for soldering large "heat sink" type PCB's, connectors, and coax cables. My guess(tm) it's 35w at best. You need something like a 60watt or higher iron, with thermostat control at 850F, and a wide chisel tip. The soldering looks like the results of trying to solder the new and disgusting lead-free solder at too low a temperature. Use a hot iron, flux if necessary, and work fast.

Yep. It's all one big solid ground (shield) around the center coax. The trick is to keep the shield end points of the balun fairly close together. Everything else can be somewhat creative. Use the photo at:

for a guide. Note that the center coax cable has no shield or shield braid inside the copper tube.

Check the dimensions of the PCB reflector. It doesn't look quite right. Hard to tell from this small a photo.

Clever idea, but un-necessary. The only thing that's really critical on the biquad is the *LENGTH* of the loop on each side. You can be very creative with the shape, but not the length. The length is measured from where the two loops hit each other plus any length added from exposed center conductor and shield conductors. The center pin connection is correct, but you've effectively added a tiny amount of length to the ground side. It's probably not critical as the antenna is fairly broadband, but it will tend to lower the optimum center frequency slightly. You can shorten both loops to compensate, but I would just shorten the ground extension and solder the two loops to ground slightly lower on the copper tube. Yeah, I know it doesn't look as aesthetically perfect that way, but it should work slightly better.

Note that it's not super critical and will probably work as you built it. There were a bunch of construction articles on biquads that totally dispensed with the copper tube coax cable extension and replaced them with a thin wire. I originally declared this to be technically disgusting and suggested that it be done correctly. I then modeled the antenna and found that it still had all of the usual

+8dBi gain, but that the VSWR was considerably higher. That will create some losses, but is not totally fatal. So, even built wrong, it will still work.

The right way:

The wrong way:

Also, the soldering in the closeup is atrocious but not unexpected considering the new lead-free solder and the cold soldering iron. Wiggle the wires and the solder connection will probably break. Try to make it look smooth and shiny. Note that mixing solder types will result in a guaranteed dull and brittle connection. Don't do it.

What are you using for insulators at the ends? It kinda looks like a dark color, which might be carbon filled nylon (anti-static). Those will work, but you really want something that is a good RF insulator, such as PTFE or coax dielectric. Nylon is hygroscopic (absorbs moisture) and will cause problems if used outdoors.

Easy on the "sir" thing. It might make me more arrogant, if that's possible.

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Jeff Liebermann

The coax is LMR-400 with outer coating and braid removed. To much hassle to remove the foil so I left it on, makes a tighter fit anyway. But since the tubing is soldered to the connector AND the reflector then can I just solder the Balun shield/braid to the copper tubing?

Have you been reading my mind? I've been pondering for days now which iron to get from this site here...

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God there are so many to choose from though! What do you like for this kind of work?

The iron in the photo is a Sears 45 W. It just about almost does it so I was thinking a nice 60 Watt should be fine, do you?

Yeh the reflector is ok. It's 123mm X 123mm

I pull real good on the connections before I sell! They're on there good but I know what you mean about being nice and shiny. Better iron will that for sure. Help me out with a good selection on a decent iron if you have a minute. And what about solder? Any recomendations?

The insulators are plastic tubing. Not sure what kind but feels like tubing you would use for your ice maker in your freezer. Twist ties coming up from bottom.

Ha Ha ok Dude! LOL


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Yep. If the foil is a tight fit, it's good enough. Soldering the balun to the copper tube is just fine. I'm a bit worried about tiny sparks from arcing or electrolysis between the aluminum and copper, but I don't think it will be a problem.

Yep. My crystal ball shows everything. I can read everything (except the documentation).

What you really should do is try to find some ecologically disgusting

60/40 or better yet, 63/37 rosin core leaded solder. When I realized that the supply is going to dry up, I stocked up with about 10 lbs of the stuff. Where it's mandatory is for repairing circuit boards that have been soldered with leaded solder. Using lead-free on that just makes a big mess.

I'm a big fan of Weller soldering irons. Unfortunately, they're also the most expensive. I have a lifetime supply of parts and pieces thanks to a failed so I don't plan to do any shopping. That means I don't have a clue what's available today.

Don't look at the power dissipation. If you must, my guess is you'll need at least 70 watts to get up to 850F. That's only for irons that don't have a thermostatically controlled tip. What you want is 750F for leaded, and 850F for lead free, preferably adjustable. Plan on buying an assortment of Weller tips at about $6/ea.

This is what I have a few of:

Way too expensive. Works fine, but uses a mechanical thermostat that requires chronic rebuilding. Not recommended unless you have lots of spare parts.

I was thinking something more like this cheapo:

I've seen these as low as $15 at ham radio swap meets. One of my friends bought a pile and hasn't complained (much) about them. It doesn't get quite hot enough, but I think it will work for everything except what you're now doing (large copper heat sinks). For the big copper stuff, get a propane torch with a big fat copper soldering tip attachment. That's what I use for the big stuff.

I do mine this way:

Yeah. Get some leaded 63/37 or 60/40 solder. If you can't get rosin core, get solid core and use some liquid rosin flux. Wash with your favorite toxic chlorinated hydrocarbon solvent and cleaner. (paint thinner works well).

It's probably ok but I can't tell for sure. Put a few in the microwave oven. If they melt, they're no good. If they just sit there, don't worry about it. Oh yeah, put a glass of water in the microwave oven when you do this to give the magnetron a load.

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Jeff Liebermann

I'll have to see how it goes but like you say I think it will be all right too'

How can you tell if the board was soldered with leaded or unleaded? My kids are always asking me to mod their game controllers and things but I never know what kind of solder to use! I'm also going to get that real skinny 12W iron just for these type of things too! What's the skinniest diameter solder I should get for this type of work? Soldering USB adapters also'

Well that page that had all the Weller irons on it has a 60W and then an 80W. Would you say try the 80 instead of the 60W?


WOW now they are sweet! I wouldn't know how to work on the thermostat! LOL

They look ok but are they toys do you think? I wonder how long they will last!

I saw that way also! I may try one of them too.

Yeh I was thinking that today about the microwave. I'll let ya know how that comes out but I think they will be ok, we'll see'

Thanks again Jeff! Talk at ya tomorrow after work.


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Two ways. One is to look for the RoHS label on the board, which always indicates lead-free. The other is to do a test soldering on a small area of the board using lead-free solder. If it looks dull gray and not shiney, the board uses lead/tin solder.

Too low wattage. You want it hot at 850F for lead-free. Lots of heat and work fast. That way you get good reflow, and a small heat affected zone.

I dunno. I have all kinds of different diameters laying around. I use them all randomly.

60 watt is about right for lead-tin. It will also work with lead-free but you'll need to take more time heating the joint. 80 watts is what methinks is right for both.

Fixing the thermostat isn't the problem. Cramming the mess back into the soldering iron handles is what generates the profanity.

Oh you're expecting reliability? At about $35, you're not going to get much of a quality product. It's adequate, I've used them, and they work. They're not the best, they don't have a LED display with the temperature, there's no desoldering attachment, and you can't install a wide range of SMT soldering/desoldering tips. In other words, it's a basic soldering iron suitable for being used and abused. If you want quality, be prepared to spend some real money. Look at Pace, Hakko, or other vendors of "soldering stations".

Incidentally, I check the power of the typical soldering pencils from these vendors. 75 or 80 watts.

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Jeff Liebermann

I also am a fan of Weller irons, I have an old EC2002 series iron which I bought at a car boot sale extremely cheaply as it required repairing. Since repair it has run for several years without problem. The manuals seem to be readily available:-

My only problem now that I have retired is that I no longer have a calibrated thermistor probe/unit to setup/calibrate the station.

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I have two EC2002 irons. One works, the other has a fried control board. A friend also has two, both with fried control boards. One of these daze, I'll reverse engineer the schematic and fix them.

Are you sure you don't mean thermocouple, not thermistor? Thermistors usually don't work above about 400F.

I use an InfraRed optical thermometer for setting up my soldering irons. I've compared it with a thermocouple plugged into my DVM and found it to be "close enough".

Reply to
Jeff Liebermann

Some of the NTC Thermistors will do upto 250C and some NTC Si Thermistors will go upto 500C.

The unit that was used for checking was owned by the electrical section,so I wasn't allowed to take it apart, and was used to check all solder stations on a yearly basis and I used to get mine done at the same time .

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Did we start talking about two different applications here because I got confused.

For the Balun and 1/4' tubing you thinks the 80 W would be better correct?

Did you also say the 12W isnb't good for the tiny work? USB adapters and the kids game controllers?

If I get a bigger iron for the tiny soldering then it's hard to find tiny tips that fit it, is that correct?

Oh wait, did you then say 80W is right for both! Both tiny and the copper tubing!?

Sorry buddy, just trying to learn more. I'm pretty good at general soldering but I didn't realize the different solders to use now!


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I'm good at doing that. I won't appologize because I have no plans to change my ways.

The balun is a tiny wire. No need for a hotter than hell iron. 750F will work with either type of solder.

However, the 1/4" copper tube and PCB are great heat sinks. You need a big tip fat chisle tip, propane torch with soldering tip, or giant electric soldering iron. It's not about how hot it gets, but about how much energy you transfer from the iron to the work. Big heat sinks require much more than small pieces of coax cable. YOu can do all that with one iron and a selection of tips and temperatures (possibly adjustable).

I have a 12w pencil iron in my truck for emergencies. If I want to wait all day for it to heat up my work, I can sorta use it. It's a PITA and nearly useless except for really tiny work.

What you want is an iron that gets really hot. Then you work VERY fast to minimize the heat affected zone. You can do it the other way around with a nearly stone cold iron, but it will take nearly forever to get the solder to melt and flow.

60 watt iron. 750F tip for lead-tin. 850F tip for lead-free. 1/16" chistle tip (or something similar) for working with PCB with conventional components. Pointed tip or tweezer tips for SMT. Two soldering irons for playing tweezer if you're lazy (like me).

Dunno. The really big irons, suitable for stained glass work, and soldering nickel steel sewing machine attachments, won't work for small PCB work. However, the common soldering station has a huge array of tips for every possible purpose.

Nope. I said that the commercial soldering stations are 75 to 80 watts. However, I again stress that it's not the wattage, but the temprature that's important. All the wattage buys you is the ability to run big tips, solder big heat sinks, and get up to temperature fast.

Argh. Stuff like this is all over the internet.

The middle photo didn't look all that wonderful.

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