G3TXQ

Amateur Radio (G3TXQ) - Wet Ladderline

Introduction

A doublet antenna fed via ladderline from a tuner is a popular solution to the requirement for a low-cost, multiband, HF antenna system. However, a question sometimes arises about losses in the ladderline, and how they change when the line is wet.

Wes Stewart, N7WS, carried out measurements on commercial 450Ω line and reported quite large increases in loss when the line was wet; his work was published in the ARRL Antenna Compendium Volume 6. In the November 2009 edition of QST magazine Joel Hallas W1ZR and Bob Allison WB1CGM published the results of related experiments and reported much smaller losses. Intrigued by the differences, I set out to conduct some experiments of my own!

Experimental method

I tried various methods to measure changes in loss, including driving the line with a known power and measuring changes in the current delivered to a load resistor. However, I found that the most repeatable method was to measure the line loss directly as an S21 measurement in a 50Ω system using a VNA2180 2-port Vector Network Analyser; then, having measured the input impedance of the line, to correct the S21 figure for the source mismatch loss. I believe this method was very similar to that used by N7WS, although his measurements were made between 50MHz and 150MHz whilst mine were made between 1MHz and 30MHz.

Of course using this method the line is not matched, but is operating with an SWR of 6:1 (300Ω) or 9:1 (450Ω); however it avoids the need for 6:1 and 9:1 baluns, and arguably is more representative of real world applications where ladderline is often operated at a high SWR. [But see later section for some matched line loss results.]

Net losses and Velocity Factor were calculated at frequences where the electrical line length was a multiple of a half-wavelength and the input resistance was a nominal 50Ω; an estimate was also made of Zo, using line input resistances measured at adjacent frequences where the electrical line length was an odd multiple of a half-wavelength.

A 1:1 current balun was used at each end of the line; each balun had a choking impedance of >4kΩ above 3.5MHz, and >8kΩ from 10MHz thru 30MHz. The baluns and the coax leads connecting them to the analyser were included as elements within the VNA2180 calibration process.

For tests where the line was elevated it was suspended in a straight line between two plastic garden chairs, with a third chair providing support at the centre to reduce the sag. At its lowest point the line was 24" above ground.

Results (300Ω line)

300 Ohm ladderline

My initial work was carried out on a 60ft length of commercial 300Ω line which I had to hand; I reasoned that the large changes that N7WS reported on his 450Ω line - if real - would likely also show up on my 300Ω line. The US manufactured line consisted of two 20AWG stranded copper conductors with black PE insulation spaced 0.275” apart. The “windows” occupied about 47% of the length.

Test 1

60ft of new 300Ω line (SWR=6:1) suspended 24" above ground:

Freq 6.68MHz 13.48MHz 20.24MHz 27.00MHz
Net loss (60ft) 0.46dB 0.57dB 0.69dB 0.71dB
Velocity Factor 0.814 0.822 0.823 0.823
Estimated Zo 289Ω 294Ω 295Ω 297Ω

Test 2

300 Ohm ladderline

60ft of 300Ω line (SWR=6:1) suspended 24" above ground and dowsed with rain water from a butt using a hand sprayer:

Freq 6.56MHz 13.16MHz 19.84MHz 26.44MHz
Net loss (60ft) 0.62dB 0.78dB 0.88dB 0.93dB
Velocity Factor 0.800 0.802 0.806 0.806
Estimated Zo 279Ω 286Ω 286Ω 287Ω

Note: a small increase in loss; a 2% drop in Velocity Factor.

Test 3

N7WS noted that “water tended to bead up and run off, making it difficult to make meaningful measurements.” I observed something similar - see Test 2 photo - although my measurements were repeatable. N7WS reckoned that a typical line used outdoors would quickly lose its water shedding ability as it degrades from sunlight and accumulates dust and other pollutants; to simulate this, he added a wetting agent to the water to create a water film on the surface. I tried to replicate what N7WS did by adding a few drops of washing-up liquid to the rain water and running the measurements again:

Freq 5.84MHz 12.44MHz 19.04MHz 25.52MHz
Net loss (60ft) 6.22dB 7.32dB 7.52dB 8.10dB
Velocity Factor 0.712 0.758 0.774 0.778
Estimated Zo 265Ω 279Ω 282Ω 280Ω

Note: large increases in loss; 5.5% drop in Velocity Factor.

Test 4

For comparison with W1ZR's experiments, I dried the 300Ω line and laid it directly in contact with the dry paving that it had been suspended above in Tests 1-3:

Freq 6.20MHz 12.56MHz 18.96MHz 24.92MHz
Net loss (60ft) 0.95dB 1.63dB 2.34dB 2.56dB
Velocity Factor 0.756 0.766 0.771 0.760
Estimated Zo 266Ω 252Ω 248Ω 262Ω

Note: large increase in losses compared to elevated line; 7.7% drop in Velocity Factor.

Test 5

60ft of 300Ω line (SWR=6:1) laid directly on the damp soil of a flower border:

Freq 6.48MHz 13.00MHz 19.56MHz 26.16MHz
Net loss (60ft) 0.97dB 1.69dB 2.27dB 2.91dB
Velocity Factor 0.790 0.792 0.795 0.797
Estimated Zo 269Ω 270Ω 265Ω 279Ω

Note: similar losses to lying on paving, but higher Zo and higher Velocity Factor.

Test 6

Out of curiosity, and spurred by the claim of a QRZ.COM forum “expert” that ladderline can be buried with impunity, I buried the line in the flower border soil at a depth of about 1":

Freq 4.88MHz 9.80MHz 15.08MHz 20.24MHz 25.96MHz
Net loss (60ft) 2.71dB 4.43dB 5.50dB 6.91dB 7.25dB
Velocity Factor 0.595 0.597 0.613 0.617 0.633
Estimated Zo 196Ω 198Ω 208Ω 212Ω 259Ω

Note: very high losses; very low Velocity Factor; low and variable Zo.

Results (450Ω line)

450 Ohm ladderline

For a more direct comparison with the work of N7WS and W1ZR, a 60ft length of 450Ω ladderline was purchased from a UK supplier. This US manufactured line consisted of two 18AWG (1mm) copper-clad-steel conductors with black insulation spaced 0.85" apart - similar to Wireman 551. The “windows” occupied about 40% of the length.

Test 7

60ft of new 450Ω line (SWR=9:1) suspended 24" above ground:

Freq 7.38MHz 14.73MHz 22.12MHz 29.51MHz
Net loss (60ft) 0.42dB 0.53dB 0.71dB 0.73dB
Velocity Factor 0.900 0.898 0.899 0.899
Estimated Zo 374Ω 365Ω 351Ω 347Ω

Note: Zo much lower than the nominal 450Ω.

Test 8

450 Ohm ladderline

60ft of 450Ω line (SWR=9:1) suspended 24" above ground and dowsed with rain water from a butt using a hand sprayer:

Freq 7.19MHz 14.44MHz 21.69MHz 28.98MHz
Net loss (60ft) 0.48dB 0.60dB 0.80dB 0.82dB
Velocity Factor 0.877 0.880 0.881 0.883
Estimated Zo 372Ω 362Ω 356Ω 344Ω

Note: small increase in loss; 2% decrease in Velocity Factor; no change in Zo.

Test 9

As Test 8, but a few drops of washing-up liquid added to the rain water to aid water retention:

Freq 7.05MHz 14.10MHz 21.30MHz 28.45MHz
Net loss (60ft) 4.24dB 4.74dB 5.06dB 5.19dB
Velocity Factor 0.860 0.860 0.866 0.867
Estimated Zo 390Ω 387Ω 386Ω 382Ω

Note: high losses; 4% drop in Velocity Factor; increase in Zo.

Test 10

60ft of dry 450Ω line (SWR=9:1) lying on dry paving:

Freq 6.38MHz 12.76MHz 19.48MHz 25.96MHz 32.68MHz
Net loss (60ft) 1.27dB 2.33dB 3.17dB 3.61dB 3.89dB
Velocity Factor 0.778 0.778 0.792 0.791 0.797
Estimated Zo 327Ω 352Ω 355Ω 340Ω 290Ω

Note: significant increase in losses, and changes in Velocity Factor.

Test 11

60ft of dry 450Ω line (SWR=9:1) suspended 24" above ground, with middle 11ft lying against aluminium ladder:

Freq 7.16MHz 14.28MHz 21.72MHz 28.92MHz
Net loss (60ft) 0.49dB 0.53dB 0.92dB 0.98dB
Velocity Factor See note See note See note See note
Estimated Zo See note See note See note See note

Note: small increase in losses at higher frequencies; TDR measurement confirmed that the 11ft section of line in contact with the aluminium ladder has a different Zo and Velocity Factor - overall figures for the whole 60ft are therefore a little meaningless.

Test 12

60ft of 450Ω line (SWR=9:1) suspended 24" above ground, during heavy rain:

Freq 6.72MHz 13.40MHz 20.12MHz 27.16MHz
Net loss (60ft) 1.08dB 1.12dB 1.66dB 1.27dB
Velocity Factor 0.819 0.817 0.818 0.828
Estimated Zo 362Ω 422Ω 373Ω 296Ω

Note: some inconsistency in the results, probably caused by the variation in rainfall during the period of the test. Results closer to those of Test 8 than Test 9

Discussion

Matched Line Loss

450 Ohm matched line loss

Subsequent to the above experiments, a set of measurements was made to determine the loss of the "450 Ohm" line under matched conditions. The input impedance of the line was measured with, firstly, a short-circuit termination, and then an open-circuit termination; from these impedance measurements the matched loss was determined and is shown by the red points on the chart. Equation (7) in the attached note on the underpinning maths was used to derive the loss, and Equation (12) to determine Zo.

The matched loss of many practical transmission lines can be modelled using three loss coefficients k0, k1 and k2, where k0 is associated with the DC resistance of the conductors (a constant for any given line type), k1 is associated with the "skin effect"(which varies in proportion to the square root of frequency) and k2 is associated with the dielectric loss (which varies directly with frequency). Dan Maguire's (AC6LA) Transmission Line Details (TLD) software uses these coefficients to predict line performance; the blue line on the chart shows TLD's predictions for line loss with k0=0.063dB/100ft, k1=0.063dB/100ft/SQRT(MHz), and k2=0. The value of k0 was chosen to match the measured DC resistance of the line - 3.5 Ohms for the 60ft length - and k1 to match the measured loss at 22MHz; the value of k2 cannot be deduced reliably from the data because its effect are relatively small in the HF range.These coefficient values produce predicted matched losses very close to the measured data; they also produce predicted unmatched line losses that accord reasonably well with the data in Table 7.

The green line shows how the predicted losses change if a DC coefficient is not included (k0=0).

By way of a "sanity check", the predicted loss of copper conductors of this size in air, with these characteristic impedances, would be 0.309dB/100ft at 22MHz, compared with a measured figure of 0.300dB/100ft and a TLD figure of 0.299dB/100ft.


450 Ohm characteristic impedance

The short-circuit and open-circuit impedance measurements were also used to calculate the characteristic impedance of the line, as shown in this second chart.


450 Ohm matched line loss

Here's how the matched loss of the "450 Ohm" line varied under a couple of different weather conditions: the blue line shows the loss during moderate rainfall; the green line shows the loss after a night of sub-zero temperatures when there was a layer of frost on the wire. The red line is the original "dry" loss.


300 Ohm matched line loss

Finally, the red points on this chart show the measured matched loss of the 300 Ohm line when dry. Zo was measured to be 295 Ohms and Vf measured 0.81.

The blue line is the loss predicted by TLD using a k0 coefficient of 0.03dB/100ft - chosen to match the measured DC resistance of the line - and a k1 coefficient of 0.095dB/100ft/SQRT(MHz) chosen to match the measured loss at 20MHz. Based on simple copper loss calculations for 20 gauge solid copper wires, expected values of k0 and k1 should be 0.032dB/100ft and 0.1db/100ft/SQRT(MHz) respectively.