Coax-Stubs.pdf

Some Q&A About Coax and Stubs for Your HF Station

By Jim Brown K9YC

Second Edition – October 2011

Q: What do I look for when buying coax for my HF station?

A: First and foremost, look for a manufacturer you can trust. Don't buy un-branded or off-

brand coax! Look for physical properties that match your use – UV resistant to stand up to

sunlight, consider weight if it will feed a flat wire dipole, look for "flooded" or armored coax

if it must lay on the ground and resist varmints. The most important electrical property is

loss, and loss below about 250 MHz is entirely the result of conductor resistance.

Q: What do RG numbers mean?

A: Not much. For all practical purposes, RG-numbers describe only the impedance and

approximate size. RG58 is a 0.2-inch diameter 50 ohm cable suitable for very short runs.

RG59 is a 75 ohm cable of about 0.25 inches diameter. RG8X is about 0.25-inch, lower loss

version of RG58, and RG6 is about 0.3-inch, lower loss version of RG59. RG8, RG213, and

RG214 are about 0.4-inch diameter 50 ohm cables; 0.4-inch 75 ohm cables are called

RG11. RG174 and RG187 are miniature (0.1-inch diameter) 50 and 75 ohm cables.

RG numbers were the original specification for coax during the military buildup for World

War II, when not much happened above 30MHz (other than radar) and the only

commercial use of radio was AM broadcasting. In those days, RG numbers did mean

something, and defined everything that mattered. That all changed in the 50s, with the

growth of FM and TV broadcasting, which brought about MATV (Master Antenna TV)

receiving systems for buildings, and CATV (community antenna TV) receiving and

distribution systems to serve entire communities. We learned how to reduce UHF loss with

foam dielectrics, and to reduce cost and weight with lightweight foil shields and

copperweld (copper coated steel) center conductors. A 1970 Belden catalog lists 14 RG59

cables; their 2006 print catalog lists 52 of them. These cables differ in many important

ways – braid or foil shields that may be copper, aluminum, or both, solid or stranded

copper center, or copper coated steel center. Their outer jacket may resist UV, or not.

Many are designed for use indoors, where their jackets and dielectrics must not burn or

create noxious fumes (the real Towering Inferno was caused by burning cables). Some have

beefy copper shields for use with broadcast video or in transmitting applications, others

have thin foil/braid shields for use in those MATV and CATV systems.

Q: Do I need cable with a "low loss" foam dielectric for my HF station?

A: Not necessarily . At HF, all that really matters is big copper, both in the center conductor

and the shield. A foam dielectric reduces loss at UHF – it provides no benefit on the HF

bands, and it costs more! However – a foam dielectric can require the use of a larger

center conductor for a given shield diameter. That larger copper reduces loss, especially on

the HF bands.

Q: Published specs for many cables don't show attenuation below 50 MHz.

A: That's because most coax is used at VHF and UHF, and because loss in coax is relatively

small at lower frequencies. For short runs on 160M and 80M, it doesn't matter. But if

you're running hundreds of feet, it can matter a lot. If there's no spec for attenuation at HF,

the spec to look for is the DC resistance (DCR) of the center conductor and the shield. Add

those two resistances together, and look for a value of 3 ohms/1000 ft or less for RG8, 7

ohms or less for RG8X.

Q: Doesn't skin effect make DC resistance irrelevant?

A: No! While skin effect concentrates current in the skin of the conductors, it is the

resistance of that skin that adds IR drop that burns power and degrades shielding. The R-

value that plugs into that equation is the resistance at the frequency of interest, and that

resistance starts with the DC resistance and increase with frequency! Bottom line –

reducing the DC resistance of that outer skin reduces loss and improves shielding!

Q:What's the relationship between size and loss?

A: The characteristic impedance of cable depends on the ratio of the diameter of the

conductors and their spacing, and the loss is directly related to the resistance. That means

we need big copper to minimize loss! Bigger coax can also handle more power, but that's

not the only reason we use it – it also provides lower loss.

Q: I'm only running 100 watts. Isn't RG8 or RG11 overkill for wire antennas?

A: It depends on the length of your feedlines, the operating frequency, and how much a dB

(or two) is worth to you. Study the graph below, which shows the loss for 100 ft of the

best coax on the HF bands. If your feedlines are 100 ft or less, loss will be less than 1dB on

40M and below, so an RG8X like LMR240, Belden 9258, or the Wireman's CQ118 are a

good choice. On the other hand, if you're running 200 ft to a tri-bander on a tower, that

smaller coax would burn more than half of your transmitter's power on 15M and 10M! If

you're using a dipole to cover all of 80/75M, the increased SWR away from resonance

increases loss. Note also that these data are for very good coax cables by major

manufacturers. Off-brand cables are often made with much less copper, and have greater

loss.

Manufacturers Published Data, LMR400 and 3227 are measured data

Q: What about copper coated steel and copper coated aluminum?

A: For frequencies below 10 MHz, avoid copper-coated steel. At higher frequencies, skin

effect takes over, and all the current is in the thin copper coating, but at lower frequencies,

the steel increases the resistance (and the loss). Measurements of LMR400, which has a

copper coated aluminum center conductor, show no increased loss due to the higher

resistance of the aluminum, even as low as 1 MHz.

Q: What about shielding?

A: The shielding provided by coax depends primarily on two factors. First, the resistance of

the shield – the lower the resistance, the better the shielding . Second, the density and

homogeneity of the shield – that is, a shield that is more dense and more uniform provides

better shielding . This is where quality of manufacture comes into play – cut into cheap coax

and you'll find relatively thin copper braid. For best shielding, look for coax that combines

a heavy copper braid with a dense layer of foil.

Q: I have a high power, multi-transmitter station. Can I use RG58 or RG8X between my

transceiver and amplifier?

A: Smaller coax cables like RG58 and RG8X provide less shielding than larger coax, thanks

to the higher resistance of their shields. To minimize inter-station interference, use coax

with a beefy copper shield for all cables.

Q: How can I use 75 ohm coax for a transmitting antenna? Aren't all transmitters 50 ohms?

A: While most transmitters and power amps are designed to match 50 ohm loads, most of

us use antennas that require a tuner to extend their bandwidth, or to load them on

additional bands. That tuner will handle 75 ohm coax just as easily as 50 ohm coax. Some

antennas are a better match to 75 ohms, and if you're trying to cover a ham band that is

broad on a percentage basis, you'll achieve lower SWR over greater bandwidth within the

antenna system with 75 ohm coax. Also, for the same copper (that is, resistance), 75 ohm

cable has about one half the loss of 50 ohm cable because the current is about one third

less. Looking at it the other way, 75 ohm coax will weigh a bit less than 50 ohm coax for

the same loss, because it requires less copper. I have two 80/40 fan dipoles up at 120 ft,

both fed with Belden 8213 (RG11 foam). In addition to reducing losses on those bands,

these antennas work quite well on 30M, 17M, 15M, 12M, and even 6M. I've made a

dozen transcontinental and KH6 QSOs on 6M with 100 watts using these antennas.

Q: Aren't RG8 and RG11 too heavy for use with flat-top wire dipoles not supported at the

center?

A: The additional weight of big coax is certainly significant, and requires that the

construction of the antenna be more robust. My high dipoles are supported between tall

trees that sway in the wind, so I use pulleys in the trees and a 100 pound weight on one

end. After several years trying various constructions, I've settled on #10 solid or stranded

copper, egg insulators at each end, and center insulators like the Alpha Delta or those sold

by The Wireman. The Budwig coaxial feedpoint attachment works electrically, but is not

easily rigged to handle the pulling stress on the wires. If you're going to use the Budwig for

this kind of antenna, you'll also need an egg in parallel with it to relieve the stress.

Q: How does moisture affect coax? What should I do about it?

A: Moisture affects coax in two important ways. First, moisture accelerates corrosion of the

braid, which degrades shielding and increases loss. Second, moisture in the dielectric (the

insulation between center conductor and shield) increases dielectric loss. To keep

moisture out of coax, all connections should be thoroughly weatherproofed.

Q: How do I keep moisture out of coax?

A: First, install connectors in a manner that minimizes the openings. If you're using solder-

type connectors, try to completely fill the holes in the center pin and the shield. When

installing cables outdoors, wrap every connection thoroughly with Scotch 88 or 33, and

cover that with a rubber mastic tape like Scotch 2228. Alternatively, tape the connection

with Scotch 88 and coat that with a product like Scotchcote. Prevent moisture from

running down the cable to the connector by putting a downward loop in the cable that

forms a "rain drip" below the elevation of the connector.

Q: I've heard that coax degrades with age, causing loss to increase drastically. Is that true?.

A: As far as performance on the HF bands is concerned, it's another one of those old

wives' tales that, while based on a grain of truth, should not be taken as gospel. What's true

are that 1) as copper braid corrodes, its resistance can increase, which increases loss and

degrades shielding; and 2) moisture in the dielectric can increase dielectric loss. However

– I recently acquired a lot of rather old coax from a neighbor who had become a silent key,

and when I tried to sell it, no one was interested. All of it was good quality RG8, RG9,

RG11, and RG213 from major manufacturers, and Mom and Dad, who had both been

through the Great Depression, taught me to be frugal. So rather than throw it away, I used

it to make stubs, and study their performance.

So far, I've built more than a dozen stubs from

this old coax, and with the exception of one

piece that was obviously of poor quality to

begin with, every one of those stubs performs

very well, including those made from a length

of RG9 (Belden 8242) that had been stored

outdoors in a shed, probably for a decade or

two (see photo at right). This is very expensive

cable – the center conductor is #13 silver-plated copper, an inner silver-plated braid

shield, a outer copper braid shield, and a non-contaminating grey jacket. That green stuff

on the outer braid is copper oxide, and even the braid without the green is a dull brown.

Note the gap between the connector and the braid – the double shields make the outer

diameter of this coax 0.424 inches, too big to fit in standard PL259 connectors. HF stubs I

built from this coax produced a very deep notch, indicating very low loss.

Why are my results so different from conventional wisdom (old wives tales)? First, because

dielectric loss is primarily a factor at UHF, not at HF, so degradation of the dielectric

doesn't matter. Resistance of the shield certainly is important, but my stub data suggests

that the real world degradation is a lot less than predicted by those old wives tales! Does

this mean that we shouldn't be careful to keep moisture out of coax? Of course not –

moisture and corrosion are a bad thing. But if I had runs of good low loss coax on my HF

antennas that were installed and trouble-free, I'd find other ways to spend my money.

Q: What's the deal with Commscope 3227 and 2427K?

A: Several years ago, a large warehouse full of this cable on 1,000 ft spools came on the

market at a fraction of its normal cost as the result of a telecom bankruptcy. Some of that

stock is still around, selling well below its normal price. It's excellent coax, equivalent in

quality and construction to the best of Belden and Times. The two types are identical,

except that 2427K , the plenum version, has a higher loss dielectric. The differences only

matter above about 300 MHz. The center conductor is #10 solid copper, so it's not very

flexible. 2427K is only 0.35-inch o.d., which makes it nice for coaxial ferrite chokes.

Q: You don't talk much about RG58 and RG59. Why not?

A: Primarily because smaller cables have more loss, thanks to the higher resistance of their

smaller diameter center conductor and higher resistance shields. The higher resistance

shields also degrades shielding. RG8X and RG6 cables are the smallest cables that make

sense in a ham station. I recently bought a mag mount antenna so I could work a network

of 1.2 GHz repeaters here in the SF Bay Area. The mount came with 12 ft of permanently

attached coax that burns 3dB (half of the transmit power) at that frequency! On 20 meters,

160 ft of a good RG58 will burn 3dB, 110 ft will do it on 10M.

Q: I've heard about low loss 75 ohm

"hard line" that is often given away by

cable TV (CATV) companies in scrap

lengths. Is this useful in my ham

station? How do I use it?

A: The half-inch size of this cable has

about the same low loss as 50Ω half-

inch hard line. It's a bit delicate – the

center conductor is 0.12-inch copper

coated Aluminum, the shield is

aluminum, and the dielectric is foam,

so it's easy to kink it if you're not very

careful. It's easy to fit PL259 connectors

onto this cable. V F depends on the

dielectric used, and is typically on the

order of 0.84.

Mfr data for some 75Ω cables, LMR400 shown

for comparison

Q: Can I use 75 ohm coax on a 50Ω antenna?

A: Sure. For monoband antennas, we can take advantage of the fact that even with a mis-

matched line, the impedance seen by a transmitter feeding a line that is a half wave long,

or some multiple of half waves, will be the actual antenna impedance. In other words, if

the line is some multiple of a half wave, it essentially disappears. To use this property,

you'll need to carefully prune the feedline to the desired length. A good way to do this is to

connect a short to one end and look for a short at the other end with an impedance bridge

like the MFJ259. If you need more length, simply add 50 ohm cable at either or both ends.

Q: How can I attach PL259s to half-inch CATV hard line?

A: Begin by removing about 3.5 inches of the outer jacket, then, strip the cable so that at

least 1 5/8-inches of the center conductor is exposed. The hard part of this is that this

cable often has a dielectric that is bonded to the shield and to the center conductor, so it is

very difficult to strip. The only good way to do this is with a coring tool. My neighbor,

K6XX, loaned me his, a Cablematic CST500, which works quite well. It's available from US

sources for about $75. You'll also want a basic tubing cutter to make the coring and

stripping go faster. Be gentle with the tubing cutter – it's easy to crush the shield if you

over tighten it. Once the center conductor is exposed, you'll need to scrape the foam that is

still clinging to it. Be careful not to scrape off the copper coating.

For each connector, you'll need two hose clamps just large enough to fit over the cable.

Slide them onto the stripped cable, then the retaining ring of the connector, then the

connector body, positioning it snug up against the end of the cable shield. Now, solder the

center conductor, slide the retaining ring over the connector body and screw it all the way

onto the body. Find a UHF barrel, mate it with the PL259, and carefully tighten it with

wrenches. Next, find some copper braid about 3/8-inch wide and cut it into two lengths of

about 1.5 inches. We'll use this braid to bond across the joint between the connector shell

and the cable shield. Using good electrical tape, carefully tape the two lengths of braid to

opposite sides of the joint, and wind multiple layers of the tape so that the outer diameter

is about the same as the diameter of the cable, going all the way to cover the connector.

Now, slide the two hose clamps over the joint, making sure that one is around connector

body and the other is around the cable shield, and tighten them carefully. Apply additional

weatherproofing to the complete joints after mating it to cables at each end.

For more great advice on using CATV hard line, including methods for pruning to length

and alternate methods for installing connectors, see QST, Jan 2000, p91, and websites by

N1BUG, N9ZIA, W9XT.

Q: So if RG numbers don't mean anything, how do I know what coax to buy?

A: First, stick with the major manufacturers – Belden, Commscope, Times, and with the

few cable vendors (Davis RF, The Wireman) who sell quality private-label cables that are

well documented on their website. Study their data sheets, and pay only for the

specifications that you need. For transmitting on the HF bands, look for beefy copper

center conductor and shields, and don't consider any cable smaller than RG8X or RG6.

Don't pay extra for a foam dielectric (unless it also gives you bigger copper), and don't buy

a cable with unpublished specs. "LMR400 equivalent" is not a specification, it's an

advertising claim, and should treated with the same mistrust as a sideshow barker! Some

high quality coax cables suitable for use on the HF bands are listed below.

RG8X : Belden 9258, Times LMR240, Wireman CQ116, CQ118 are approximately equal.

RG8 : Lowest loss – Commscope 3227, 2427K, Times LMR400UF, LMR400; Belden 8268.

RG8 : Slightly greater loss – Belden 8267, Belden 9913, Wireman CQ1000,

RG8 for stubs: Belden 8237 or 9251, Commscope 3227 or 2427K; Davis RF 213.

RG11 : Lowest loss – Belden 8213;

RG11 : More loss – Belden 9212

RG6 : Lowest loss – Belden 8238, 8261

Q: What are some good types of coax for my HF station?

A: The table below lists a known good cables with full published specifications from

known good vendors. Costs listed are those I found with internet searches in Feb 2010

from reputable vendors. Shorter lengths are more expensive per foot. Note that RG58 and

RG59 are not on this list – they have too much loss for long runs, and their shielding is not

as good as the larger cables. As I have time, I plan to add some RG6 cables to the list. For

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