Part 10
Welcome to Part 10 of our discussion.
We'll talk a little more about Coaxial Transmission Line, aka Coax Cable.
This month's topic is prompted in part by another reader's question.
Bubba-Joe, WE1LID asks:
"My SWR's were really high, about 1.5:1. One of the old-timers in the club suggested injecting my coax with something he called DiHydrogen Monoxide (DHMO). He mentioned that it's hard to find, but that he had some on hand that he could sell to me at a good price. I bought a quart jar of it from him for $10. Following his instructions, I carefully injected it into my coax at 1-foot intervals. Like magic, my SWR's disappeared. But now I'm having trouble hearing signals. I bought an amplifier, but that didn't help. What do you think is wrong with my radio?
73's, Bubba-Joe, WE1LID
Well Bubba-Joe, you've managed to roll quite a few mistakes into one question. Let me help you out here...
1. You should NEVER, EVER use an apostrophe to pluralize something. (I admit, not radio related, but one of my pet peeves).
2. SWR and 73 are singular, not plural. You only have one Standing Wave Ratio at a single measurement point. 73 means "Best Wishes". Does 73s then mean "Best Wisheses"? (Two more pet peeves that ARE radio related).
3. A 1.5:1 SWR is not "really high". In fact, it's perfectly acceptable. Most modern radios will begin to fold back power when the SWR gets above about 2:1. Especially at HF frequencies, the extra loss introduced into the antenna system by a 1:5:1 SWR is not worth worrying about. But if you really want to get it perfect, knock yourself out. Then watch what happens when it rains, the leaves fall off the trees, or you replace your old aluminum siding with vinyl.
4. An amplifier does absolutely nothing to improve received signals. Spend your money on improving your antenna system. It's far and away the best bang for the buck in upgrading your station. As the QRP guys say, "power is no substitute for skill -- and a good antenna"!
5. DHMO is actually available quite easily. Its chemical symbol is H2O. You paid about 40 bucks a gallon for water. I would have been happy to sell it to you for half of that price <g>.
Okay, I'll 'fess up here. Bubba-Joe does not exist, at least as far as I know. But now you know not to say "seven-threes" or write 73's. More importantly, the point remains -- water is the mortal enemy of your coax. And it's particularly insidious because the harm accumulates slowly, so you don't notice it. As water invades your feedline, it becomes more and more lossy. And that has the perverse effect of making your SWR look better and better! Why? Remember from our earlier discussions -- SWR is the ratio of forward to reflected power. When you measure the SWR in the shack, you're still seeing full power going up the coax. But the water is absorbing and dissipating a lot of that power before it ever gets to the antenna. That also means a lot less power that can get reflected back down the coax, making your SWR look better -- and a lot less of your precious RF that gets radiated as a signal. Many, if not most, hams today use an automatic antenna tuner, either built into the rig or as an external device. We push the tune button, get the green light, and off we go. Even with a manual tuner, the day-to-day difference is so small that we don't notice it.
That's why waterproofing our connections is so important. If there's a way for water to get in, believe me, it will. Even in nearly ideal conditions, as our coax slightly expands and contracts as it heats and cools from day to night, some moisture from the air gets pulled in. If your coax has been up in the air for a while, I recommend that you test it. Here's an easy way to do it, using simple instruments that you may already own or can readily borrow, or purchase for not a large sum.
You'll need:
Your transceiver
A short 50 Ohm Coax jumper, 1.5 - 3 feet at most
An SWR/Power meter suitable for the frequency and power level you intend to test at
A 50 Ohm dummy load suitable for the frequency and power level you intend to test at
And of course the coax you want to test.
Consult your owner's manual for expected power output for the mode/band you have selected. DO NOT USE SSB!!! Sideband will only put out full power on hard voice peaks. BE CAREFUL IF YOU SELECT CW!!! Some rigs will indicate transmit when you push the mic's PTT button but will not actually transmit a carrier. In that case, you must have a key (or a shortable plug) installed in the key jack. DOUBLE CHECK IF YOU SELECT AM!!! Many/most radios will only generate a 25W carrier in AM. Many/most radios WILL put out full rated power in FM or possibly RTTY mode. Be sure you know what you should expect to see on the meter before you assume there is a problem!!
If you can perform the test at VHF or UHF, the losses in the coax will become easier to see. Just be sure that your test equipment is suitable for the band. You don't need laboratory grade equipment -- we're not as interested in absolute measurements as we are in relative measurements. In other words -- what we measure in test #1 versus test #2 is more important than that what the actual values are relative to the measured values. Hint: VHF/UHF dummy loads are not real common, but you can make a quick and dirty half-wave dipole with some wire and a panel or bulkhead SO-239 connector that will do just fine hung a few feet off the ground and in the clear. Make sure to ID any transmissions.
Refer to the following diagram to perform the test (you may notice that art and drawing are not among my stronger skills). Get the manufacturer's loss specifications for the coax you are testing. This will be a "matched loss" figure -- i.e., the coax is terminated in a resistive load equal to its characteristic impedance. Simply -- if it's 50 Ohm coax and you use a 50 Ohm dummy load, you're all set. You can use the chart from part 8 of this series (scroll down a bit to see it) to get a good estimate. Remember that the loss figures are per hundred feet. Let's take a hypothetical example. We want to test a 50' length of RG-8X. From the chart, 100' at 150 MHz = 4.5 dB matched loss. That means 50' has 2.25 dB loss. Actually it will be very slightly less since we'll test at about 146 MHz, but the difference is negligible.
We'll take a little detour here, into the land of the mysterious decibel. I won't get into a long involved explanation about decibels. Particularly in working with antennas and feedline, they are quite useful, and I recommend that you have at least a working knowledge of them. For now, what you need to know are two very simple formulas. 1. Gain or loss in decibels (dB) = 10 x log(power1/power2). For example -- if we raise our transmitted power from 50 Watts to 100 Watts, 10 x log(100/50) = 3.01 dB. Note that it's the ratio of the two power levels that determines the answer. Raising power from 500W to 1000W is exactly the same 3.01 dB increase. Similarly, reducing power from 100W to 50W = 10 x log(50/100) = -3.01 dB. 2. Sometimes (as in our feedline test) we need to convert dB to a percentage. The formula to do that is % = 10^(dB/10), or 10dB/10 -- that's 10 raised to the power of the dB gain/loss divided by 10. Remember that if you're calculating a loss, you need to enter the dB figure as negative. That gives us what we need to conduct our feedline test. 3. If you know your way around a spread sheet, even a little bit, you can enter those formulas and make a handy chart for quick reference. I know that Windows has a built-in scientific calculator, and presume that Apple does as well. Apps for your Smart Phone or tablet should be available too. And perhaps not surprisingly, most major search engines will calculate the answer if you enter the equation in the search box. 4. Quick facts to remember -- 3 dB means either double or half; 6 dB means either quadruple or a quarter; 10 dB means 10 times, or a tenth.
Using the second equation, and our expected loss of 2.25 dB in 50 feet of RG-8X:
% = 10^(-2.25/10)*10 = 59.56%. In other words, we should see about 60% of our original power at the far end of the coax.
Set things up as shown in test #1. If you have set up the radio correctly, you should see very close to the specified power on your meter when you key the rig. Note: make sure that your meter is set to read forward power.
Now switch the position of the meter to the other end of the coax, as shown in Test #2. Don't change any settings on the meter. If we started with a 100W carrier, we expect to see about 60W now. If it's significantly less, we know that our coax needs to be replaced. What's "significant"? That's up to you. I certainly would not be reaching for the MasterCard if it showed 59 Watts. But 29 Watts? You bet I'd replace it! Using the first equation, our total loss is
10 x log(29/100) = 5.4 dB Note that 60W is 2.25 dB loss. If additional losses cut the power about in half (to 29W) the loss is about 3dB more. Surprise! 3 dB is half of your power. See how handy those dBs (NOT dB's!!) can be? Finally -- remember that it's a two-way street. Losses in your coax attenuate incoming signals just as well. Ya can't work 'em if ya can't hear 'em.
That's it for this month. Next time, we'll continue our explorations. Until then,
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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