RG-59 flexible coaxial cable composed of:
A: outer plastic sheathB: woven copper shield
C: inner dielectric insulator
D: copper core
May 29, 2012
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Feedline Frenzy
Which Kind to Use? How to Measure? Tips & Tricks
Overview
As hams we regularly deal with moving RF around the shack and (especially) out to antennas in distant locations - the back yard, the attic, up in a tree, nailed to the peak of the house, snaked along the gutter ... you name it! And many times we have multiple antennas out there, each with precious signals needing to find their way back into our fortified bunkers in undisclosed locations.
But how many of us are familiar with the many varied types of coax out there for doing all this? Sure you know RG8 and RG58. And if you've been a QRPer for any amount of time you know about RG-174. But at what power level, length of the run, or frequencies of the signal is it necessary to graduate up to using the next size? What other kinds of feedlines are out these at the major suppliers that might be better/cheaper for your application? And how about all those different characteristics, like velocity factor, dialectric strength and "suitable for buried use"? Man, the questions keep on coming!
So in this week's session of CWTD we'll be overviewing the "basics" of the "feedline state-of-the-art" in order to give participants some insight and valuable references for this technology that we use every day in the shack. Then as is also our style, we'll present a working project that can be used to illustrate the principles under discussion in a very practical and useful manner. You'll love this week's project, which will span a couple of weeks and end up with an appliance that we've each wanted to have for some time. We think you will want one too!
73, George N2APB & Joe N2CX
Audio Recording ... (Listen to the MP3 podcast)
Discussion Notes:
<20:08:38> "George - N2APB": I'm going to get my next
antenna up, which is an EFZ fed by a ladder line leading up the side of the
house.
<20:10:49> "George - N2APB": Talking about balanced vs unbalanced ... good to
explore as far as connecting to antten on one end, and to the ATU on the other.
<20:12:04> "George - N2APB": Open wire is well-banced, inherently low loss, and
pretty good for use in high SWR settings <gasp!>
<20:12:46> "George - N2APB": But matching the balanced to unbalanced conditions
is important ... balans to balance and match impedances.
<20:13:31> "George - N2APB": Q: Is "zip line" (aka, speaker wire, power cords) a
good feedline?
<20:14:31> "George - N2APB": Plays havoc with the music being played. (Along the
wire.)
<20:15:01> "George - N2APB": Impedance can be calculated ... "There's a table
for that."
<20:16:24> "George - N2APB": "I use RG-174 for everything ... even for getting
out to the back 40 of the yard ... nice and small so it can be supported well
and easy."
<20:19:51> "Joe N2CX": I use RG-174 quite a bit as well. For jumper cables in
the shack it's very convenient. Handy too for feeding an antenna keeping lenghts
at 30 feet or less keeps loss tolerable.
<20:21:54> "Joe N2CX": The chart is an eye chart cleverly disguised as a data
chart...
<20:22:55> "Joe N2CX": 8.5 dB att 100 MHz!
<20:34:11> "George - N2APB": Which is better ... high or low Velocity Factor ...
and why?
<20:35:37> "George - N2APB": How does "percent shield" affect performance?
<20:37:21> "WA0ITP Terry": Joe do you crimp the 174 connectors or solder them?
<20:41:19> "George - N2APB": Ahhh, so Velocity Factor is just a factor to
consider when trying to get the right length
<20:44:46> "Joe N2CX": 9913 has half the loss (dB) of 9914 for the same diameter
coax cable
<20:46:17> "WA0ITP Terry": Ladder line to "tuner", 8x into shack at the home qth,
no feedline portable
<20:47:23> "Ted WA3AER": Wa3AER Ted: RG-213 and 9913 for HF, LMR-400 for VHF/UHF
<20:51:37> "WA0ITP Terry": EFZ - Thats what I use, it works well autotuner is n
basement
<20:52:28> "Joe N2CX": Zepp is short for zepplin - half wave end-fed cuz at LF
Zepplin couldn't feeed antenna at center!
<20:55:08> "Al - N8WQ": that project sounds great!
<20:57:21> "Ted WA3AER": This would be a boon for expanding the capbility of the
window panel I have...when implemented, seemed 4 ports was PLENTY...NOT!!
<20:59:24> "George - N2APB": Ha! You said it Ted. At one time I thought a 2.5"
of PVC was ample for bringing cables in/out of the house. Add in
<21:01:23> "Pete - WB2QLL": However, parallel coax when operated off its design
impedance will have very high loss, not like open wire.
<21:11:26> "N5AB Bill": an hour goes fast here!
<21:14:33> "Armand WA1UQO": Looking forward to the development of this project -
Have the Rookie kit and the box already. Will be a great addition to this shack!
Thanks again for a great presentation.
<21:14:41> "WA0ITP Terry": Thanks Guys
SESSION NOTES
Feedline Frenzy
The Basics
… Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880.[1] Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line.
RG-59 flexible coaxial cable composed of:
A: outer plastic sheath
Discussion Outline
- Different kinds of feedlines: open wire, ladder line(s), coax
- Characteristics of each: impedance, power, mounting (e.g., alongside of house, in-ground, elevated), velocity factor, losses, cost, …
- Common coax types we use: RG8, RG58, Beldon xxxx, RG174,
- Tricks that can be used: stubs for matching, terminate far end and measure for quality,
- Coax connectors: BNC, N, PL259/SO239,
- Bringing feedlines into the house
- Switching feedlines (coax switches)
- Good coax vendors
- Problems with older coax feedlines
- Sealing connectors against moisture
"DATA CHARTS, TABLES & PLOTS"
Coaxial Cable Loss in db per 100ft.
Coax Data Charts ... from N0HR page ... http://www.n0hr.com/hamradio/104/10/ham_radio0.htm
Coax data - attenuation, velocity factor, impedance, OD for various types of coax.
Coax Data Attenuation - db/100 feet Belden # Impedance 100 MHz 400 MHz 1000 MHz OD V Factor 9880 50 1.3 2.8 4.5 .390 .82 This is Thicknet Ethernet cable. Most is marked "Style 1478" and has a #12 solid center conductor and 4 shields (2 braid/2 foil). Attenuation - db/100 feet Belden # Impedance 100 MHz 400 MHz 1000 MHz OD V Factor 8240 50 4.9 11.5 20 .195 .66 8267 50 2.2 4.7 8 .405 .66 8208 50 9 .405 .66 9258 50 3.7 8 12.8 .242 .78 9913 50 1.3 2.8 4.5 .405 .82 9914 50 9 .403 .66 Attenuation - db/100 feet Hardline Impedance 100 MHz 400 MHz 1000 MHz OD V Factor 1/2 50 0.8 1.8 3.0 .500 .66 3/4 50 0.66 1.49 2.4 .750 .66 7/8 50 0.55 1.3 2.3 .875 .66 Attenuation - db/100 feet RG # Impedance 100 MHz 400 MHz 1000 MHz OD V Factor 4 /U 50 5 B/U 50 .332 .66 8 /U 50 1.8 4.7 6.9 .405 .66 8 A/U 50 9 .405 .66 8 /X 50 3.7 8 12.8 .242 .78 10 A/U 50 .475 .66 28 A/U 50 44 /U 50 45 /U 50 46 /U 50 47 /U 50 .625 .66 58 A/U 50 4.9 11.5 20 .195 .66 58 C/U 50 4.9 11.5 20 .195 .66 60 /U 50 74 /U 50 .615 76 /U 50 87 A/U 50 .425 88 /U 50 90 /U 50 91 /U 50 92 /U 50 93 /U 50 94 /U 50 .5 95 /U 50 96 /U 50 97 /U 50 98 /U 50 99 /U 50 115 /U 50 .375 116 /U 50 .49 118 /U 50 .78 119 /U 50 .465 120 /U 50 121 /U 50 122 /U 50 7 15.2 25 .16 .66 126 /U 50 .28 128 /U 50 141 /U 50 3.2 6.9 13 .19 142 /U 50 3.9 8.2 13.5 .206 143 /U 50 .325 156 /U 50 .54 157 /U 50 .725 165 /U 50 .41 174 /U 50 8.9 17.5 28.2 .101 .66 188 A/U 50 9.8 15.8 25 .11 .66 190 /U 50 .7 196 A/U 50 9.8 15.8 25 .08 .66 209 /U 50 .75 211 /U 50 .73 212 /U 50 1.6 3.6 8.8 .336 .66 213 /U 50 2.2 4.7 8 .405 .66 214 /U 50 2.2 4.7 8 .425 .66 215 /U 50 2.2 4.6 9 .475 .66 217 /U 50 1.4 3.1 5.8 .545 .66 218 /U 50 .81 1.9 3.8 .87 .66 219 /U 50 .81 1.9 3.8 .87 .66 220 /U 50 .7 1.5 3.5 1.12 .66 221 /U 50 .7 1.5 3.5 1.195 .66 223 /U 50 4.5 9.2 14.3 .212 .66 224 /U 50 1.5 3 6 .615 225 /U 50 7.5 .43 226 /U 50 .5 227 /U 50 .49 228 /U 50 .795 280 /U 50 .48 281 /U 50 .75 301 /U 50 .245 303 /U 50 9.8 15.8 25 .17 .66 304 /U 50 .28 316 /U 50 10.4 16.5 31 .102 .66 393 /U 50 2.1 4.4 7.5 .36 400 /U 50 3.1 8.1 13 .171 403 /U 50 13.6 26.5 45 .116 404 /U 50 16.3 32.4 68 .116 405 /U 50 22 .085
Coaxial Cable Power Ratings
Maximum input power rating - Watts at (MHz)
RG/U CABLE | 1.0 | 10 | 50 | 100 | 200 | 400 | 900 | 1000 | 3000 | 5000 |
---|---|---|---|---|---|---|---|---|---|---|
55,6A,212 | 4000 | 1500 | 800 | 550 | 360 | 250 | 150 | 65 | 50 | |
8 MINI,8X | 4000 | 1500 | 800 | 550 | 360 | 250 | 150 | 65 | 50 | |
8,8A,10A,213 | 11000 | 3500 | 1500 | 975 | 685 | 450 | 230 | 115 | 70 | |
9913,9086,9096 | 3500 | 1500 | 975 | 685 | 450 | 230 | 115 | 70 | ||
4XL8IIA,FLEXI 4XL | 3500 | 1500 | 975 | 685 | 450 | 230 | 115 | 70 | ||
9095 | 11000 | 3500 | 1500 | 975 | 685 | 450 | 230 | 115 | 70 | |
9,9A,9B,214 | 9000 | 2700 | 1120 | 780 | 550 | 360 | 200 | 100 | 60 | |
11,11A,12,12A, 13,13A,216 |
8000 | 2500 | 1000 | 690 | 490 | 340 | 200 | 100 | 60 | |
14,14A,217 | 20000 | 6000 | 2400 | 1600 | 1000 | 680 | 380 | 170 | 110 | |
17,17A,18,18A, 218,219 |
50000 | 14000 | 5400 | 3600 | 2300 | 1400 | 780 | 360 | 230 | |
55B,223 | 5600 | 1700 | 700 | 480 | 320 | 215 | 120 | 60 | 40 | |
58 | 3500 | 1000 | 450 | 300 | 200 | 135 | 80 | 40 | 20 | |
58A,58C | 3200 | 1000 | 425 | 290 | 190 | 105 | 60 | 25 | 20 | |
59,59B | 3900 | 1200 | 540 | 270 | 270 | 185 | 110 | 50 | 30 | |
62,62A,71A,71B | 4500 | 1400 | 630 | 440 | 320 | 230 | 140 | 65 | 40 | |
62B | 3800 | 1350 | 600 | 410 | 285 | 195 | 110 | 50 | 31 | |
141,141A,400 142,142A |
19000 | 9000 | 3500 | 2400 | 1600 | 1100 | 650 | 350 | 245 | |
174 | 1000 | 350 | 160 | 80 | 80 | 60 | 35 | 15 | 10 | |
178B,196A | 1300 | 640 | 330 | 240 | 180 | 120 | 75 | 40 | - | |
188A,316 | 1500 | 770 | 480 | 400 | 325 | 275 | 150 | 80 | 53 | |
179B | 3000 | 1400 | 750 | 480 | 420 | 320 | 190 | 100 | 73 | |
393,235 | 25000 | 9500 | 6300 | 4300 | 2800 | 1700 | 880 | 620 | ||
402 | 9000 | 3500 | 2400 | 1600 | 1100 | 650 | 350 | 245 | ||
405 | 130 | |||||||||
LDF4-50A | 19000 | 6100 | 2600 | 1880 | 1310 | 906 | 563 | 551 | 294 | 217 |
LDF5-50A | 44000 | 7700 | 7740 | 5380 | 3720 | 2550 | 1620 | 1520 | 785 | 568 |
MEASUREMENT TECHNIQUES
1) Use a scope to measure the length and impedance of coax ... Alan Wolke, W2AEW
This video (http://www.youtube.com/watch?v=Il_eju4D_TM) shows one way to use a scope and function generator to measure the length of a piece of coax transmission line as well as estimate its impedance. It uses a "poor man's TDR" type of measurement by launching a pulse into the coax and measuring how long it takes to return after being reflected by the open circuit end. This same technique can be used to determine the distance to a fault (open or short). A simple method for determining the impedance of the line is also shown.
This video touches briefly on transmission line and reflection theory, but is definitely not intended to dive deep into these topics. There are literally books written about this topic - so that won't be covered here.
2) DIY Time Domain Reflectometer (TDR) ... Juha Niinikoski OH2NLT
A valuable and simple tool for determining cable length & impedance measurements and cable system fault finding is with the Time Domain Reflectometer, or "TDR". A homebrew TDR can be built with oscilloscope and pulse source.
http://en.wikipedia.org/wiki/Time-domain_reflectometer
and
http://www.epanorama.net/circuits/tdr.html
Some time ago I played with the TDR concept and was surprised how accurately the cable impedance can be measured with this method. TDR measurement is also a valuable tool when you make phasing loops (cable delay lines) for particular antenna system.
NOTE: The following coax measurement techniques employ the use of an "antenna analyzer" such as the MFJ-259, the Autek RF-1, or the Micro908 Antenna Analyst which is actually used in the discussion. (The analyzer shown in the photos is the Palstar ZM-30, a derivative design of the Micro908.)
3) Transmission Line Characteristic Impedance
The characteristic impedance of coaxial, twisted pair, open wire or ribbon type feedlines can be estimated using the Micro908. Practical measurements are best done in the mid-tuning range of the instrument where accuracy is optimum and feedline lengths are reasonable; so this procedure will be performed between 7 and 21 MHz. The measurements need to be done with a transmission line over frequencies where the feedline is at about 1/8 wavelength at the low frequency end and something over ¼ wavelength at the high frequency end, so it is recommended that a length of about 16 feet is used.
Connect the near end of the feedline to the Micro908. Connect a 1000-ohm carbon or Cermet potentiometer to the far end with leads no longer than an inch or so. Initially set the pot to its highest value. See Figure 8. Ensure that the transmission line is supported for its entire length in a fairly straight line and kept several inches from any conductive surface or material. This is important to minimize any detuning effects. Ideally the line should be dressed along to top of a wooden fence or supported by fiber rope or string.
Now tune the Micro908 over the range of 7- to-21 MHz while noting the
resistive (R) and reactive (X) values. More than likely they will vary
widely over the tuning range. Now readjust the potentiometer to a slightly
lower value and do another sweep while observing the variation of R and X
values. At some potentiometer setting the R value will vary very little over
the tuning range while the X value will remain near zero. This is the
estimated characteristic resistance.
4) Transmission Line Loss
Transmission line loss for 50-ohm feedlines can be easily measured using the analyzer. The basic operating principle is that loss in transmission lines attenuates RF sent through them. When the line is connected to the analyzer and the far end is short or opencircuited there is a theoretically infinite SWR. If the feedline had zero loss this would be the case. However since any real line has some loss both the forward and reflected power are attenuated and a finite SWR is measured. For most good quality new coaxial feedlines the loss at HF frequencies will not exceed several dB per hundred feet; however as they age the dielectric becomes lossy to it is a good idea to periodically check the loss.
Measurement is simple. All you have to do it is to remove the load, short-circuit the far end of the feedline, and then connect the near end to the analyzer’s RF output connector. Measure the SWR and refer to Table 1 for the approximate corresponding loss. If the measured SWR is above 9:1 that’s good news since the SWR then is less than 1 dB. If you vary the analyzer frequency you will see that SWR decreases with frequency indicating that loss increases at higher frequencies.
Table 1 – SWR vs line loss (infinite load SWR)
Approx Loss Measured SWR 1 dB 9:1 2 dB 4.5:1 3 dB 3:1 4 dB 2:1 5 dB 2.3:1 6 dB 1.7:1 7 dB 1.6:1 8 dB 1.5:1 9 dB 1.4:1 10 dB 1.3:1
5) Transmission Line Stub Lengths
Measurement of quarter and half wave transmission line stubs can be performed regardless of the transmission line characteristic impedance. The method relies on the fact that an open-circuited quarter wavelength line or a short-circuited line acts like a precise short circuit at the chosen frequency of operation.
With either type of feedline first cut it about 10% longer than the desired length, taking the appropriate velocity factor into account. The velocity factor of common feedlines is available from manufacturer’s literature or references such as the ARRL Antenna Book. If you cannot find the value or if you are using a custom type of feedline, the “Velocity Factor Measurement” section in this manual provides a way to determine this value. The following formulas can be used to estimate the length of transmission line required.
For a half-wavelength stub the length is: L = (5904 * VF) / F
Where L is the length in inches, VF is the velocity factor and F is the operating frequency in MHz for the stub. Similarly for a quarter-wave stub use the formula: L = (2952 * VF) / F
To determine the length of a half wave stub, connect the near end of the transmission line through a 51-ohm resistor as shown in Figure 4 to the analyzer’s RF output connector. Short circuit the two leads at the far end of the half wave stub.
Ensure that the transmission line is supported for its entire length in a fairly straight line and kept several inches from any conductive surface or material. This is important to minimize any detuning effects. Ideally the line should be dressed along to top of a wooden fence or supported by fiber rope or string.
Now tune the Micro908 for minimum SWR and note the frequency. This is the frequency where the transmission line is exactly a half wavelength long. If the initial length was chosen properly it should be below the desired frequency. If so, cut off a short length making sure the far end is still short-circuited, and repeat until resonance is achieved at the desired frequency.
For a quarter wave stub, the above procedure can be used except, of course that the length is different and that the far end needs to be open circuited.
Transmission Line Velocity Factor
Velocity factor of a transmission line can be measured using techniques similar to the ones used for measuring quarter and half wave stubs.
The procedure can be performed at any frequency that the Micro908 tunes but it is most practical in the vicinity of 10 MHz where line lengths are reasonable and instrument accuracy is optimum.
Either a quarter wave or half wave length can be used; but using the shorter length consumes less feedline if it will be discarded after the measurement.
Begin by cutting a quarter wavelength of feedline using the formula: L = (2952 * VF) / F for a frequency of 10 MHz and assuming a VF (Velocity Factor) of 1.
Now connect the near end of the feedline to a 51-ohm resistor as shown in Figure 9 then to the analyzer’s RF output connector. The far end must be open circuited.
Ensure that the transmission line is supported for its entire length in a fairly straight line and kept several inches from any conductive surface or material. This is important to minimize any detuning effects. Ideally the line should be dressed along to top of a wooden fence or supported by fiber rope or string.
Now tune the Micro908 for lowest SWR and note the frequency. VF can now be calculated using the formula: VF = 10/F, where F is the measured frequency in MHz.
THE PROJECT
Project goals
· Remotely select any one of four antennas over a single coax feedline
· Control head located in shack with rig
◦ Rotary switch to select antenna
◦ Needs DC power source of 12V @ 200 ma.
· Control done by passing all signals over feedline via triplexer
◦ Normal transmit/receive RF
◦ Pulsed audio to select one of four relays
◦ DC power for remote controller and relays
· Remote controller where antenna feedlines are present
◦ Suitably weatherproofed
◦ Triplexer separates out three signals
▪ RF
▪ DC power for controller
▪ Audio signaling for controller
◦ Controller decodes audio signalling
▪ Operates one of four relays for antenna selection
▪ Default (no power) no antenna selected
· Operation up to several hundred feet of coax cable
· Usable over HF ham bands
· Will handle up to at least 25W of RF
· Minimal SWR degradation
· Less than 0.5 dB transmit/receive loss
· Hardware based on NJQRP Rookey remote controller
◦ Simplicity, reliability
◦ Design re-use
Weatherproof enclosure
Pix of box, overall about 6" x 6" x 4" ...
Pix of box interior ...
Pix of box cover with gasket ...
· Standard NEMA outdoor plastic case
· Readily available
· Rugged, designed to last
· Gasketted cover
· Mounting ears
· 6”x6”x4” size
◦ Plenty of interior room for electronics
▪ Rookey controller board
· LEDs to show which relay selected
▪ Triplexer/relay board
◦ Allows for RF connector mounting
▪ BNC
▪ UHF
▪ ???
· Coax cable entry
◦ Connectors on bottom of box
◦ Common chassis connectors
▪ Not waterproof but ok on box bottom
◦ Coax-seal ™ or self amalgamating tape to seal cable connectors
◦ Additional hole on bottom
▪ Box ventilation
▪ Allows condensation to escape
▪ Mesh over hole keeps out water “splash”
The design will evolve ... and will continue next week!
REFERENCES
1) Feedline/Coax Suppliers ...
The Wireman ... http://thewireman.com/coax.html
Beldon ... http://belden.com/
Cable Experts ... http://www.cablexperts.com/cfdocs/cat.cfm?ItemGroup=6&itmsub=0&bskt=0&USA_ship=1&c=0
davis RF ... http://www.davisrf.com/amateur.php
RF Connections ... http://therfc.com/coax.htm
2) Tools and Resources
Soldering PL-259 Coax Plugs -- From W8WWV
Soldering The Dreaded PL-259 -- From SolderIt
How to solder PL-259’s compiled by K1VR
Assembling RG-174 BNC Connectors
Making Crimp-On coax connectors for RG-213 - Nicely illustrated - from Paul, VE7BZ
Soldering PL-259 Coax Plugs -- From W8WWV
Soldering The Dreaded PL-259 -- From SolderIt
How to solder PL-259’s compiled by K1VR
How to Make RF Cables - From Green Bay Professional Packet Radio
Assembling RG-174 BNC Connectors
Unsoldering And Recovering Old Electronic Components -- From IZ7ATH, Talino Tribuzio
Coax Sealtm -- Hand-moldable, plastic mastic, a waterproof, long-lasting seal for coaxial cable