Feedline Loss vs. Antenna Height
Very often I hear hams complaining about the losses in coax cable, and ways to reduce it (or avoid it altogether). Unfortunately, some of the methods suggested often result in poorer overall performance.
One example is the idea of putting the antenna at a lower height, to keep the coax loss to a minimum. Usually this only reduces performance.
To look at this issue, I considered a portable dipole antenna for the 20m band, fed with RG-316 or RG-174 coax: thin 2.5 mm (0.1 inch) 50 ohm coax cables, of the type that I use for my backpack dipole antenna kit. While there are lossier types of feedline, this is pretty much of a worst case, with about 0.1 dB loss per meter (double the loss of RG-58 coax), so serves as a good example.
I modeled the 20m dipole at various heights from 2m ( 7 feet) to 20m (66 feet) above the ground, in 2m steps, and looked at the gain at elevation angles of 10 degrees and 30 degrees, a typical range for many ham contacts. I reduced the modeled gain by the loss of length of transmission line required to reach the ground.
|effective gain at 30 degrees, dBi||effective gain at 10 degrees, dBi||feedline loss dB|
|2||( 7 )||-1.1||-9.3||0.2|
|4||( 13 )||2.3||-5.6||0.4|
|6||( 20 )||4.5||-3.4||0.6|
|8||( 26 )||5.1||-1.3||0.8|
|10||( 33 )||6.0||0.8||1.0|
|12||( 39 )||6.2||2.5||1.2|
|14||( 46 )||4.7||3.2||1.4|
|16||( 52 )||1.9||3.4||1.6|
|18||( 59 )||-2.1||3.7||1.8|
|20||( 66 )||-9.1||4.3||2.0|
If we look at the gain at 10 degrees, which is a reasonable value for many DX paths, the performance continues to improve with height, in spite of the feedline loss. Even with 2 dB of feedline loss, the higher antenna is still better than for any lower height.
Lowering your antenna due to concerns about feedline loss, at least in the range of cases considered here, always degrades performance.
Now we see a different pattern at 30 degrees: the gain peaks at about 12m height and then drops down to a low value at 20m height. This is not due to the feedline loss, but due to the the vertical radiation pattern of the antenna. Even without considering feedline loss, maximum gain is around 12m, and at 20m there is a null in the pattern very close to 30 degrees, resulting in the drop at that height.
If we want to maximize performance for a given path and feedline type, mount the antenna at the optimum height for the path, without worrying about the feedline loss.
That is not to say that using a better feedline won’t improve performance: it will, by the difference in the feedline loss. But if weight, availability, or other factors limit your choice of feedline, there is no reason that should limit the antenna height due to the feedline loss. Feedline that runs horizontally, however, doesn’t provide the advantage of added antenna height, so it is worthwhile trying to minimize it.
This analysis can be performed for other bands and feedline types, but the conclusions don’t change much, at least at HF, and probably for reasonable heights at VHF as well.
For my HF backpack dipole antenna kit, I carry about 8m (25 ft) of RG-174 coax. That’s a reasonable length for the typical heights I’m going to use while on the trail (with a bit extra to reach inside the tent), as a trade-off for the weight of the coax. Each person has to make their own trade-off decisions in that regard.