A review of QST article "The Lure of Ladder Line"

Analysis

General

The antenna described in the QST article is 20.1m (66') long, at a height of 9.1m (30') above ground, and fed in the centre with 15.2m (50') of coax / ladder line.

The antenna model discussed in this article is consistent with the above, and additionally, ground &epsilon=13, σ=0.05, conductor 3mm (1/8") diameter, and RG-213/U / Wireman 551 ladder line. A lossless balun is assumed.

This model also includes a practical L match. The L match used for tuner loss is in general, the most efficient way to transform the load impedance. Using the loss of a practical L-match tends to underestimate the loss in any other tuner configuration.

The NEC model deck is available.

This analysis does not consider directivity, or polar pattern of the radiator, or degradation of ladder line performance when wet.

Coaxial feed

Steve was looking for a low impact, multi-band HF antenna and had a notion that "Perhaps I could string up a single dipole and feed it with coaxial cable, using an antenna tuner to tune it on several bands." He implemented such a system and used it on 40m up to 10m, citing 75 new countries for a DXCC as evidence that it "worked". 

Steve's article used RG-8/U coax which has a nominal Zo of 52Ω. This analysis uses the more popular 50Ω RG213/U, but losses are very similar to RG-8/U.

Figure 1 shows the results of modelling the coax feed configuration using 15.2m (50') of RG-213/U. The graph shows the principal components of antenna system loss over the frequency range of 1 to 30Mhz. The losses reported here are much higher than in the QST article, eg line loss of 17.2dB at 3.8MHz against QST's 13.7dB.

Fig 1: Antenna system losses with RG-213/U feed

Clearly, this configuration has very high losses on all bands except 7MHz and 21MHz.

A similar coax feed arrangement using the lossier RG-58C/U is shown in Figure 1a.

Fig 1a: Antenna system losses with RG-58C/U feed

This configuration has all the same problems of the RG-213/U feed in Figure 1, just that it is even worse.

The coax feed performance highlights that:

• an SWR meter and antenna tuner cannot fix, and may hide problems that exist later in the antenna system; and
• the need to view the antenna system as a system, and to not focus only on some components.

Ladder Line feed

On the advice of a peer, Steve then explored the same antenna with 15.2m (50') of ladder line. The article presents some predicted feed line losses at spot frequencies in the main HF bands.

Figure 2 shows the results of modelling the ladder line feed configuration using 15.2m (50') of Wireman 551. The graph shows the principal components of antenna system loss over the frequency range of 1 to 30Mhz. The losses reported here are much higher than in the QST article, eg line loss of 5.4dB at 3.8MHz against QST's 1.37dB.

Notwithstanding comments in the reference article, the configuration does not have acceptable losses on 80m and 160m. Antenna system loss on 1.8MHz is 19.6dB, and on 3.6MHz is 6.5dB.

The configuration does provide quite acceptable multi-band antenna system loss on all HF amateur bands from 60m to 10m.

The radiator behaves as a low dipole. At frequencies where it is relatively short, the pattern is nearly omni directional, and the pattern transitions to multiple lobes and nulls at frequencies where the dipole it is multiple wavelengths long.

Figure 2 shows the principal components of antenna system loss over the frequency range of 1 to 30Mhz.

Fig 2: Antenna system losses with Wireman 551 feed

Clearly, this configuration has very high losses on bands below 5Mhz, but is quite acceptable at and above 5MHz.

Critics have suggested that selection of Wireman 551 (the "original" ladder line) exaggerates losses. Figure 3 shows the principal components of antenna system loss over the frequency range of 1 to 30Mhz for Wireman's most expensive ladder line, #554. The modelled losses are slightly better with 554, but for example, the improvement of 0.03dB at 7MHz is so slight that it is not readily visible in the chart, and is not reason alone to pay more than twice the price.

Fig 3: Antenna system losses with Wireman 554 feed

Line loss inconsistencies

Inconsistencies were noticed between line losses reported in the QST article, those calculated here, and published line loss specifications.

The QST article gives a ladder line loss for what should be close to a 75+j0 load at 7.15MHz of 0.07dB which is much better than:

• the 0.13dB theoretical prediction for two bare 1.3mm (#16) copper conductors spaced 19mm (3/4") (ladder line conductors with air dielectric), or
• the 0.16dB prediction for Wireman 552 (1.3mm (#16) copper conductors spaced 19mm (3/4")).

The QST ladder line losses seem overly optimistic, perhaps they forgot to allow for the additional loss under high VSWR, or their method of accounting for mismatch is flawed.

There is a reference in the text to the RG-8/U coax having loss of less than 1.5dB/100' according to the ARRL Handbook chart (whereas the matched line loss is more like 1.9dB/100'). This error would flow into any calculation of load under mismatched conditions.

A later QST article in April 1994 "Get out your calculators or computers" picks up on the earlier article and changes the configuration, which muddies the waters a little. They report a modelled feed point impedance at 3.8MHz of 10.3-j878.8 and 100' of ladder line having a line loss of 2.2dB. Again, using N7WS's characterisation of Wireman 551, the line loss would be 4.5dB. The same article quotes a line loss of 18.1dB using RG-213/U whereas using Belden's published figures for 8267 (RG-213/U), the line loss would be 15.8dB.

Fig 4

The differences seem to be in the characterisation of the lines (coax and ladder line), and in the transmission line loss models that were employed.

Figure 4 shows a comparison of loss models for four open wire lines:

• W551: Wes Stewart's characterisation of Wireman 551 (19mm / 1.02mm or 3/4" / #16) ladder line;
• O470: theoretical loss of air dielectric 470Ω (25mm / 1.02mm or 1" / #18) (approx same conductors as ARRL "Generic 450Ω ladder line");
• O400: theoretical loss of air dielectric 400Ω (19mm / 1.30mm or 3/4" / #16) (approx same conductors as W551);
• ARRL: Extrapolation of ARRL Antenna Handbook 18th Ed characterisation of "Generic 450Ω ladder line" (25mm / 1.02mm or 1" / #18).

It is unlikely that the ARRL's generic ladder line (450Ω ladder line (25mm / 1.02mm or 1" / #18) has loss lower than the theoretical losses for an air dielectric line with the same conductors. The conductors for a 19mm spaced line need to be at least 2mm diameter (#12) to deliver the loss that ARRL specifies. Perhaps the ARRL handbook is not a good reference for matched line loss, and further scope for error occurs where the loss under high VSWR is estimated.

Conclusions

• Using an antenna tuner to extend a 20.1m (66') dipole with 15.2m (50') of coax feed (essentially a 40m λ/2 dipole) for multi-band operation will have unacceptably high feed system losses on most HF bands. A VSWR meter between transmitter and tuner does not give indication of this problem, and indeed hides the problem.
• Although 15.2m (50') ladder line makes the 20.1m (66') dipole more useful as a multi-band antenna than using common coax feed line of the same length, the antenna system is not an acceptable "all-band" antenna. Its feed system losses below 60m are unacceptable. There are a number of ways to extend the lower limit, and the simplest of those is to lengthen the dipole if the space is available. The article Feeding a G5RV models the performance of a 31m (102') dipole with open wire feeders and shows acceptable losses down to 80m where it is  0.36λ long.
• The results here are specific to the model, and care needs to be exercised in extending them to other scenarios. In particular, increasing the feed line length will increase losses, and have the greatest effect when the losses are already significant. Some assumptions, such as the lossless balun, are further challenges in multi-band operation of this type of antenna system.
• Ladder line is a good product, but it is not the panacea for all problems. It's (matched line) loss is relatively low, but it is usually operated at high to very high VSWR which causes significant additional loss depending on the specific circumstances. In these models , the 5dB loss in 15.2m (50') of ladder line as used in this antenna at 3.8MHz illustrates the point. It is not a case of  "Simply throw it up in the air and let the tuner worry about providing a low SWR for the transceiver" as suggested by Steve in a later article entitled The Classic Multiband Dipole Antenna.
• The ARRL characterisation of "Generic 450Ω ladder line" indicates much less loss than Wes Stewart's characterisation of the Wireman products. The ARRL Antenna Book 18th Ed describes "Generic 450Ω ladder line" as #18 conductors spaced 1", and yet shows losses much lower than the theoretical copper-only losses of a transmission line of those dimensions. 

Links

• The Lure of Ladder Line.
• Wes Stewart's (N7WS) analysis and comments on the same QST article are interesting. It is Wes's characterisation of the loss of Wireman ladder lines that is used in the models for this article.
• Get out your calculators or computers
• The Classic Multiband Dipole Antenna
• Feeding a G5RV.
• Transmission Line Loss Calculator Enhanced for prediction of transmission line loss under mismatched conditions.

How was it done?

The analysis is based on an NEC model of the radiator. The NEC model of the dipole is available here. NEC2 was called from a custom Perl script that created the Sommerfield ground model and run NEC2 to create a report of feed point impedances from 1MHz to 30MHz in 0.1MHz steps.

A Perl script was written to parse the NEC report, and do the transmission line calculations (impedance transformation, loss, voltage etc) and L-Tuner simulation. The results were written to a tab delimited text file which was loaded into Excel for the final analysis and graphics.

Transmission line parameters come from the Transmission Line Loss Calculator / Transmission Line Loss Calculator Enhanced.

(Note that none of the results depend on approximations based on VSWR.)

Changes

V1.03 30/04/08 15:52:49 -0600 .

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

VK1OD on the 'net. I appreciate your feedback. © Copyright: Owen Duffy 1995, 2007. All rights reserved. Disclaimer.