What Shielded wire to use for RF Test Leads

Looking on the web, I have found numerous suggestions and no definite answer.

To make a shielded test lead for use on a RF device, such as a Signal generator, Oscilloscope, or other, I have seen recommendations for TV coax (RG59), CB/Ham antenna wire (RG58) and microphone cable. Which is best?

I know that the TV cable is stiff, with that solid wire in the center, so that would not be the ideal cable to use. It's been years since I used the CB/Ham cable. I know that stuff is stoffer too, but maybe not as bad. As far as handling and softness, I know the Mic cable is probably the best, but I read that it's probably the least desireable because of impedance issues.

So what is normally used?

BTW:

  • I might be wrong on those RG numbers. I'm just going on my failing memory.
Reply to
oldschool
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For RF, the best is the cheapest !

A tube of aspirine for instance with a hole on the top for the lead.

I use this for my scope probe, it is perfect.

Reply to
Look165

I have used the stranded coax, RG58AU, RG174,

Greg

Reply to
gregz

All depends on the type of testing. For RF I usually use some rg-400. It is slightly less than 1/4 of an inch in diameter ( same size as the rg-58) That is double shielded silver plated and a stranded center wire. But that is because I am often doing duplex work and need the isolation of the double shielding. RG 58 is fine if you do not need the extra shielding. Just get the kind with the stranded center conductor.

Usualy rg 174 for light weight leads. It is about 1/8 of an inch in diameter. I am not sure of the number but there is a teflon and silver cable made in the same size.

All that is for a 50 ohm system or audio work.

Most scopes will use a probe with a 10/1 on it. You can get them from the China places off ebay for about 15 to 20 dollars per pair. I bought a set and compaired them to some HP probes rated at 100 MHz and they seem to be about the same. Not sure how long they will last, but at that price should be good for a while if not mistreated. Without the

10/1 probe you will often load down a circuit with the scope leads. The cable will have about 30 pf of capacitance per foot. I am thinking the scope only has about a 1 meg ohm of impedance without the 10/1 probe.
Reply to
Ralph Mowery

(...)

I use RG-58c/u for 50 ohm test leads. The stranded center wire is more flexible than the rather stiff RG-58a/u.

For 75 ohm RF, I use RG-6/u double shielded CATV cable with F connectors on both ends. If I need to plug into test equipment, I have female F to Male BNC adapater.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

I've never heard of RG58 C/U. Where would someone buy a small amount of it (Online)? Looking on Ebay, I see the A/U type. And that stuff is mostly pre-made cables with either Pl-259 , or BNC connectors on it, OR large rolls of the bulk wire. (25 ft is the shortest). I could buy a 12ft with BNCs, and cut it in half to make two leads, but that's the A/U type.

I'm thinking this is something that might need to be ordered from a store that sells Ham Radio stuff, but do they sell small lengths and do they ship it?

Reply to
oldschool

Also known as Belden 8262.

30ft for $36 including shipping and BNC connectors on the ends. A little pricey but about what I would expect for a long test lead.

It's what's used on test leads and mobile antenna coax cables. Bend the solid center conductor RG-58a/u a few times and it will break. Try to get the type that has both foil and wire braid shields.

Kindly disclose how many feet of the stuff you want and I'll find it for you. How many feet in a "large roll of bulk wire"? eBay is a good place to get short lengths, but not high quality cables.

How many feet and I'll find you a source. It's quite common.

Also, you'll need mating BNC, UHF, whatever connectors. I avoid solder type connectors because they like to fall apart. Crimp type are stronger and more reliable. You'll need a crimper:

100ft for $42 plus shipping:
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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

Honestly, we havent; the foggiest of what you want to do.

The generator probably has a 50 ohm output. If it's an FM radio, the input is likely 75 ohms. So, you do need a matching transformer. So, there's ge nerally BNC to BNC and an F connector at the receiver. You can buy adapter s to adapt the BNC to F. or BNC to quick F.

RG-58 isn't terribly inflexible. If you want flexibility, then RG174 shoul d work.

At one point, at work we had lots of crimped BNC cables, bought from a repu table manufacturer. Nearly all failed at one point. the broken ones were eventually fitted with clamp BNC plugs. They are not the easiest things to put together, but a resistance soldering tool made it a lot easier.

It also depends where you want to put the adapters.

Reply to
Ron D.

Nope. Most of my test equipment is 50 ohms. However, I also work with 75 ohm devices and antennas. Transformers do not have a very flat frequency response. If I need a flat frequency response from DC to many GHz, I use a minimum loss pad: etc... Figure on 5.71dB loss. The technique also works for 50 to

300, 50 to 600 ohms, and other impedances.

Yep. Coax adapters are fun:

More:

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

I have used some of the six hole ferrite beads to make transformers. six turns, tapped at the fifth turn. The 1.2 turns ratio, squared is a

1.44:1 impedance ratio. These are the same beads used for CATV line taps and broadband splitters.

They convert 75 ohms to 52 ohms. I put some into some old Blonder Tongue inline attenuator housings with an F connect on the 75 ohm end and a BNC connector on the 52 ohm end. (52.0833 Ohms) They were handy to use a TV FSM as a 50 ohm RF millivoltmeter. Now, I have some precision attenuators, and a Boonton 9200 RF Millivoltmeter ans different feedthrough terminators.. :)

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Reply to
Michael A. Terrell

** The dopey OP has one of these:

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The output Z is anyone's guess.

He only wants to use it for old, tube AM radios.

IMO use no cable at all, just let it radiate via a length of wire.

.... Phil

Reply to
Phil Allison

A/U is stranded center too. Rg58 is solid. Au cu difference seems to be outer covering difference.

Greg

Reply to
gregz

Oops, you're right. I stand corrected:

RG-58a/u (Belden 8259) PVC jacket UL temp temp rating = 75C Attenuation at 1GHz = 70.5dB

RG-58c/u (Belden 8262) Non-contaminating PVC jacket UL temp temp rating = 85C Attenuation at 1GHz = 74.8dB

So, different jackets, higher temperature rating, and less loss for RG-58c/u. Also, RG-58c/u appears to have a military designation (MIL-C-17, M17/155-00001), while RG-58a/u does not.

More: Q: I see different jackets types advertised, what are the differences, and why I should care?

A:Jacket Types: PVC-I Polyvinyl Chloride (PVC), black (contaminating). Contaminating jackets will breakdown from sunlight (Ultra-Violet Resistant), and cannot be buried. The PVC-I is rated with a shorter life cycle than PVC-IIA. Suitable for low grade jumpers.

PVC-IIA Polyvinyl Chloride (PVC), black (non-contaminating). Non-Contaminating jackets will not breakdown from sunlight (Ultra-Violet Resistant), and suitable for direct burial without conduit. The PVC-IIA is rated with a twenty-year life cycle. The best all-around jacket. (etc...)

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I initially did the same thing using various iron and ferrite toroid cores. They worked, but not if I wanted a really flat frequency response (+/-0.5dB) over many octaves of frequency range. I had a few adapters that I optimized for frequencies of interest, but getting it fairly flat from 1.5 to 30 MHz (marine bands at about 4.5 octaves) was rather difficult. Instead of fighting the problem, I switched to a minimum loss pad. The low frequency pads ended up inside Pomona aluminum boxes, while the microwave stuff was on microstrip stuffed into a box made from brass or unetched PCB material.

Actually, it wasn't quite a minimum loss pad. I worked out the numbers for a 50 to 75 ohm -6.0dB pad instead of -5.71dB, which made power and voltage scaling, measurements, and calculations easier.

One problem with minimum loss pads. They develop feet and walk away. I wasted about a day of company time building and characterizing a collection of about 20 attenuators for my own use. Within about 8 weeks, they were all gone, probably "borrowed" by my co-workers.

Hint: There's no such thing as a precision attenuator in a lab that works with transmitters. Eventually, they all get cooked. The best I could do was characterize what was left of the attenuator after someone accidentally transmit into it, and compensate for the changes in loss and impedance.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

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