CWTD May 29, 2012

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

[From Wikipedia] … 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
B: woven copper shield
C: inner dielectric insulator
D: copper core

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.