Category Archives: Radio Field Craft

Comparing Coaxial Loss in RG-58 and RG-316

by Eric, WD8RIF

I recently decided that I would operate portable during the upcoming ARRL 10 Meter Contest and would combine this operation with a pair of Parks on the Air (POTA) activations, one on the Saturday of the contest and one on Sunday. While putting together my field station for this, I realized I wanted to see how lossy my RG-58 and RG-316 feedline cables were on 10m. Yes, I could have resorted to online charts of feedline losses, and I know that the RG-316 is lossier than the RG-58, but I thought it would be fun and more enlightening to make actual measurements, to empirically determine the losses in my particular coaxial cables.

The two coaxial cables in question are both 25′ in length and both are equipped with BNC male connectors on each end. The RG-58 cable is a high-quality cable that was originally manufactured to be a 10Base2 ethernet cable, but it’s probably now over twenty years old. The RG-316 cable was recently purchased from Tufteln (link) and includes an RF choke near one end.

To make my measurements, I used my Elecraft KX2 (link) to generate RF into an Elecraft DL1 dummy load with RF detector (link), using a digital multimeter to measure the voltage at the DL1’s measurement points. I used the formula that came with my DL1 assembly instructions to calculate the measured wattage:
P = (((V x 1.414) + 0.15))^2)/50

I made measurements in the CW portion of the 10, 15, 20, 40, and 80m bands, with the KX2 set at 5 Watts output.

My first set of measurements was made with the DL1 connected directly to the KX2’s antenna jack using a BNC union:

Direct (no feedline)
Band Volts Watts
10 11.62 5.498
15 11.59 5.470
20 11.61 5.489
40 11.63 5.508
80 11.63 5.508

My immediate observation was that the KX2 appears to be generating more than 5w when it is set to be producing 5w but also that the KX2 output is essentially same from 10m through 80m.

My second set of measurements was made with the DL1 connected to the KX2 through the RG-58 cable:

Direct RG-58
Band Watts Watts
10 5.498 4.809
15 5.470 4.844
20 5.489 5.004
40 5.508 5.184
80 5.508 5.276

My third set of measurements was made with the DL1 connected to the KX2 through the RG-316 cable:

Direct RG-316
Band Watts Watts
10 5.498 4.190
15 5.470 4.322
20 5.489 4.507
40 5.508 4.774
80 5.508 4.959

I was pleased to see to see that both the RG-58 and RG-316 behaved as I expected them to: the loss increased with increasing frequency.

Using the magic of MS-Excel, I created a table of Loss in Watts for both cables, relative to direct connection and to each other:

Direct Difference (Loss) (Watts)
Band Watts RG-58 vs
RG-316 vs
RG-316 vs
10 5.498 0.690 1.309 0.619
15 5.470 0.626 1.148 0.522
20 5.489 0.485 0.982 0.497
40 5.508 0.324 0.734 0.410
80 5.508 0.232 0.549 0.317

In looking at this table, it was immediately obvious that RG-316 is much lossier than the RG-58, particularly on 10m. By looking at the row for 10m, one can see that I am losing nearly 0.7 watts in the RG-58 but I am losing over 1.3 watts in the RG-316.

For completeness, I added columns for Loss in dB to the spreadsheet:

Difference (Loss) (dB)
Band RG-58 vs direct RG-316 vs direct RG-316 vs RG-58
10 0.582 1.180 0.598
15 0.528 1.023 0.495
20 0.402 0.856 0.454
40 0.263 0.621 0.358
80 0.187 0.456 0.269

This exercise showed me that for my upcoming ARRL 10 Meter Contest POTA outings, I would do best by connecting my antenna directly to my transceiver, if possible, without using either coaxial feedline. If conditions at the operating site require me to use feedline, I will chose the RG-58 over the RG-316.

The tables also tell me that RG-316 is pretty lossy regardless of the band; for my regular field operating, unless I’m planning to do bicycle- or pedestrian-portable operations where weight and bulk is a consideration, I’ll carry RG-58 instead of RG-316. (I purchased the RG-316 specifically for bicycle- and pedestrian-portable operations, and I plan to continue to use the RG-316 for those applications.)

At some point, I will repeat this exercise with RG-8X, a feedline that is very close in size to RG-58, is less lossy, but is also heavier and stiffer.

Do I allow antenna wires to touch tree branches during field activations?

Many thanks to Keysrawk on my YouTube channel, who asks:

Do you usually try to use an isolator or do you often let your wires touch branches by just pulling them over? When you deploy 20m EFHWs, for example, do you try to avoid having an end touch a branch and only have the throw line going over the branch? I tried to go through your videos and look but you don’t often mention how far you pull the wire up and possibly over. Thanks!

This is a great question!

Before I answer, I’d like to add a little context:

  1. I am a QRP operator. The maximum amount of power I use in the field is 10 watts, but 99.5% of the time, it’s actually 5 watts or even much less.
  2. I am answering this as a field operator, meaning I’ll be referring to temporary antenna deployments.

That said, the quick answer is no, during park and summit activations, I do not worry about my antenna radiator wire touching tree branches.

I do isolate the end of my wire antennas from tree branches and leaves, but I don’t worry about other parts of the radiator touching.

Also, all of my antenna wire has some sort of jacket–I don’t run bare wire in the field.

More often than not, when I deploy a longer wire antenna–say, a 40M EFHW–I simply use a tree branch to support the apex of the antenna if I deploy it in an inverted vee configuration. Continue reading Do I allow antenna wires to touch tree branches during field activations?

Radio Field Craft: Rand explores the handy Prusik Knot

Many thanks to Rand (W7UDT) who shares the following guest post:

The Prusik Knot… strain relief for Wire Antennas and Coax.

By W7UDT, Randall ‘Rand’ Tom

The Prusik knot is a simple, yet effective means to provide needed strain relief to wire antennas and coax, while deployed. It’s comprised of a simple loop or length of suitable cordage (of smaller diameter than the shank, Coax or Wire, it secures).

The link below, is from our friends at Animated Knots…. It’s a brief video tutorial on how best to tie the Prusik knot. Having the Prusik in your ‘bag of tricks,’ will help make your next field deployment be a successful one.

As seen in the instructional video, the knot is comprised of a simple loop of cordage, sufficient in length for the task, which is fine, but, I would recommend NOT making a loop. Rather, keeping tag ends for easier anchoring. These tag ends should be at least 12” in length. This is called an open-ended Prusik. Either way, both have utility.

To do this, fold a 24” of cordage in half, to make a ‘bite.’ Lay the bite over the shank of the coax or wire, and feed the tag ends inside the bite loop. This forms a larks-head knot. Wrap two additional turns around and through, then dress and test the knot to form the Prusik. Simple. Anchor (tie) the tag ends at a point where strain relief is best positioned. Then adjust (slide) it to load.

After deployment, I would also recommend leaving the Prusik attached. It comes in handy, when coiling your feed lines or elements later for proper storage.

The Prusik allows it to slide along the wire or coax while free of tension, yet it holds fast under load. Much like a monkey’s fist hanging onto a vine. The tag ends, can then be affixed to suitable anchor where needed. The key here, is using a smaller diameter cordage, than the wire or coax itself.

The Prusik, along with similar ‘Friction Hitches’, can be used in any number of applications in Ham Radio. e.g. Anchoring coax, joining linked antenna elements, power cords, and adjustable guying. The list is long, wherever strain relief is needed.

I would encourage you to tie it, try it, test it, and judge for yourselves. As well, I would encourage you to check out other useful climbing friction hitches… YouTube is a great place to start. I hope you find this useful afield, and to hear you ‘On the Air!’

73! de W7UDT (dit dit)

W7UDT, ‘Rand’, lives and operates near Boise Idaho, with his lovely wife Stacy. Portable QRP operations, along with his Jeep and Harley are his ‘vices.’ Your comments and questions are welcomed. My email is [email protected].