So, as I’ve reported, I’ve put up a Cobra antenna, which is a version of the non-resonant doublet antenna, and am tuning it with an old Johnson Matchbox antenna tuner. The procedure for adjusting the tuner is to first set the band switch to the appropriate band (the Matchbox only has positions for 80m, 40m, 20m, 15m, and 10m). Next, you adjust the Tuning control and the Matching control for minimum SWR.
These two controls interact with one another. So, while you first adjust the Tuning control for a minimum SWR reading and then adjust the Matching control, you may have to actually detune the Tuning control to get the lowest overall SWR.
I’m adjusting the tuner by connecting the output to my antenna analyzer, and the lowest SWR that I can currently obtain on 80m is 1.5:1.
Here’s question #1. A 1.5:1 SWR results from a number of different resistance (R) and reactance (X) combinations. The question is should I maximize R or minimize X when adjusting the tuner? I’ve been minimizing X. Do you think that’s the right thing to do?
The Icom IC-746PRO that I’m driving this with has an internal antenna tuner. Question #2 is whether I should engage this internal tuner to tune out that final bit of SWR so that the rig sees a 1:1 SWR. My guess is that it probably doesn’t matter that much, but I thought I’d ask.
WATSA, OMs?
Minimizing X seems like a good approach. Reactive loads are not fun. A 100 Ohm resistive load should cause less problems than a 50 + 20j Ohm load.
Modern solid-state power amps are generally happy into a 2:1 SWR. But it would probably reduce component stress to dial out that last bit. I can’t see how it would hurt, unless the reactance is really high. Then you could get some exciting see-saw current between the two tuning units.
If things are dodgy, you can always back off to 50W or 25W. That gives more safe headroom for the amp components. The operator on the other end probably won’t notice.
I’m certainly not a specialist in transmission lines — I was happy to get a C+ in my fields and waves class. But my EE degree specialization was in signals and systems, so you can take this with as much salt as you want.
Dan,
1) Since reactance is seen as a resistance by AC (and radio frequency energy is AC), I think you’re on the right track there.
2) I would keep the 746’s tuner in bypass mode – no need for two.
Dan,
When in doubt, back check QST. From the Jan 1995 edition: http://www.arrl.org/files/file/Technology/tis/info/pdf/9501046.pdf
Checking the Matchbox circuit, I think you’ll find it to be a “T-Network”. Day to day I am using an old Heathket SA=2040, so this article is where I started to get to know my tuner.
Good luck.
BTW, in setting up your tuning “chart”, I find it best to attempt to have the Cout setting to be relatively the same from band to band. That way, when changing freq/band, retuning is very quick as you adjust the inductor and then quickly set the Cin knob. Reality gets in the way, but when you are chasing a multiplier in a contest, you need to be quick.
That’s a good idea, but on this old Matchbox, the Tuning control knob tends to loosen up, throwing off all those settings. I need to figure out how to fix that. Thanks for the link to the QST article.
Dan,
Here is another article specifically referencing the design of the Johnson Matchbox. It is a different design than the typical T-Network. Might be worth a look: http://www.w4btx.com/priceless_tuner.html
A lot of modern solid state transmitters start to fold back at 1.5:1 VSWR. Since you are “on the edge”, it’s likely best to engage the tuner in your radio. I habitually use the tuner in my K3 to tune out SWR’s on my quad that fall in the 1:6:1 range and lower, just because it means that the output stage is looking into a better match, than if I don’t. I realize it is not affecting anything on the cable going from the transmitter to the antenna, but if I can maximize the radio’s efficiency a little, it’s a win.
Secondly, if you have ever tuned an old tube transmitter that had “Tune” and “Load” controls, you may recall they interacted. This was mainly due to the fact that as the Load was increased, it changed the impedance seen by the final tank circuit (controlled by the Tune control), thus requiring some back and forth touch up.
Essentially the same thing applies to your matchbox. As you vary the Matching control, it reflects a varying impedance back into the Tuning portion of the circuit, thus the interaction you see.
As far as minimizing R vs. X, the “best” match will be the one that brings the absolute magnitude of Z (which is sqrt(R^2 + X^2) the closest to 50 ohms, or whatever the design impedance the transmitter is designed to drive.
Remember that all of this applies at a specific frequency, or at least a very narrow band (in terms of % of the frequency) of frequencies. Just like a dipole ‘cut to length’, it will only show a non-reactive match at a single frequency. As you move away from that frequency, the impedance gains a complex term, either plus or minus, depending on the direction of movement. A matchbox (and transmitter internal ‘tuner’) will likewise only ‘match’ at one frequency or narrow band of frequencies at a time, and will require touch up as you move around the bands. The waveform on the transmission line changes accordingly, also.
You should get reactance to zero, and if there are a number of combinations of the tuning and matching controls that do that, use the one for which the ratio between resistance and 50 ohms is the closest to 1. Ratio closest to 1, not difference closest to 0. 75/50 = 1.5 and 50/30 = 1.67 so 75 is closer to 50 than 30 is, in this sense of “closer”.
But a better solution might be to try a different length of feedline. Often if a high SWR feedline presents an impedance hard to match with your antenna tuner, a different length of feedline will present essentially the same SWR but with a different combination of R and X which the tuner can handle better. What you’re doing is moving around a circle on the Smith chart. Your tuner may like some parts of the circle better than others. Try a slightly longer feedline and if the best you can do is worse than what you had before, then what the system wants is a shorter length. Of course it has to be long enough to reach! Don’t coil any extra feedline up.
Yes, I do plan to cut off about ten feet of the feedline and see if that makes any difference. I get such a nice match on 30m and 40m now, though, that I’ve been putting that off. My friend, Thom, W8TAM, did this with his doublet. He made it quick connect/disconnect by putting PowerPoles on the ends.