SWR assumptions

Discussion in 'Amateur Radio News' started by VK6FLAB, Apr 28, 2018.

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1. VK6FLABHam MemberQRZ Page

SWR assumptions

In the past I've talked about the Standing Wave Ratio, the SWR, and how it describes some of the characteristics of your antenna system. I say system because it's not just the antenna, it's the connection between your radio and the antenna as well. The coax or feed line, their length and how you've connected your antenna, all feature in the performance of the entire kit and caboodle.

As an aside, that's why measuring an antenna with an SWR meter at the bottom of the antenna, while you're bolting it to the top of your mast is likely to give you a different result when compared with the measurement performed at the radio.

During the week I was asked about how cutting an antenna changes the SWR. The question included a quote from the ARRL Single-Band Dipoles page which states:

""If you see that the SWR is getting lower as you move lower in frequency, your antenna is too long. Trim a couple of inches from each end and try again.""

The person asking the question, Phil, wanted to know why he was seeing a different behaviour.

I've seen the same myself and until I had the benefit of an antenna analyser it also made little sense to me.

The reason it makes little sense becomes clear once you realise what assumptions you're working under.

When you look for antennas online, or when you buy one, often it comes with a lovely SWR graph. You'll see frequencies on the horizontal axis and SWR on the vertical axis. You'll likely see a lovely mostly horizontal line with a dip downwards at the frequency where you want to use this antenna.

The assumption you will almost automatically make, I know I did, for years, was that outside the graph the line continues on its merry way in both directions. That means that you're assuming that the SWR comes down in one place and the rest of the time it's high.

With the benefit of an antenna analyser you can graph the whole HF spectrum, and depending on the hardware, you might even be able to see VHF and UHF or higher.

One thing you'll immediately see is that the SWR is all over the place. It's up, and down, crazy lines, across the whole spectrum. You'll find enormous highs and some very interesting lows along the way.

It's one reason why I can use an antenna intended for the 10m band on the 2m band.

When you're making an antenna, like a single-band dipole, you might find yourself in a position where your antenna SWR is going up and down like a yo-yo around the frequency where you're wanting to be. The higher the frequency, the more likely that your trimming ends you in a different dip or a different high, outside the one that you're actually looking for.

One other comment. The ARRL quote which is talking about HF dipoles states that you should remove a couple of inches from each end. Let's take that literally, two inches from each end, that's 4 inches in total. Let's call it 10cm between friends. If you're trimming a dipole for 160m, you'll change the frequency by just over 1 kHz, but if you're doing this on 6m, then the same trimming will change the frequency by nearly 1 MHz and if you use that HF recommendation for 2m, the change is almost 6 MHz, so, trimming a couple of inches as the ARRL suggests will work for some dipoles on some frequencies, but might get you completely crazy results for other frequencies.

Now you know, the SWR isn't high across everything except where you care, it's all over the place and sometimes that helps, and sometimes it doesn't.

I'm Onno VK6FLAB

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VU3JNM, KK5R, KI7SIJ and 1 other person like this.
2. KA4AQMXML SubscriberQRZ Page

Here’s a suggestion for newer hams and for others who just didn’t know....never ‘trim’ each leg of your simple dipole.

Once you have your dipole strung out for an initial SWR measurement ( you gotta start somewhere, right?) and you determine that it needs to be shortened for example, just fold your excess wire (after the end has passed through the insulator) back on the antenna and twist it snuggly. Take another measure and go from there. Whatever you fold back, remember to do the same on the other leg. The closer you get to ideal, the easier it will be to get more power where it needs to be. By using this fold back method, it reaches you antenna characteristics big time. Fold back (read as excess or needed antenna wire) at the insulator ends at 28 MHz will be quite different from 3.8 Mhz. Play with it, learn from it and your days of ‘trimming’ wire will be over.

BTW, I use 14 gauge, brown, twisted wire that I buy in 250 foot roll from big box hardware stores.

GL

Randy
KA4AQM
Chesapeake Va

KK5R likes this.
3. KG5THGPlatinum SubscriberPlatinum SubscriberQRZ Page

Also, always check the swr of the antenna system at its full time operating height and location . It's so tempting to make adjustments where it's easy to work with.

KK5R, K1FBI and SA6CKE like this.

Never underestimate the value of a well-tuned antenna system. Putting the effort and money into an antenna can make a huge difference in performance. Both transmission and reception will be a lot better.

With antenna system it's also the inclusion of the cabling. Don't underestimate cable loss. I have made an Excel spreadsheet that allows for calculation of the radiated power depending on input power, cabling, antenna gain and SWR that I have found useful. Can be downloaded here: http://www.bedug.com/radio/ErpCalc.xls

Never forget that losses cuts both ways - both transmission and reception, even though on the reception side the signal/noise ratio also plays a role the sensitivity of the receiver is also a factor.

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5. K3KICHam MemberQRZ Page

Investing in a good antenna analyzer with a graphical display saves times and can also be very educational. Well worth the price.

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6. KZ5RSubscriberQRZ Page

Folding back the wire on a dipole is an excellent method with one caveat, if after tuning the dipole you decide to remove the excess wire the tuning will change. Cutting the excess eliminates the capacitive coupling created by the wire being folded back against itself and consequently the tuning of the dipole. Just wrap it tight, leave in place and enjoy.

I would also like to emphasize K3KIC's comment and take it one step further to say it is an essential piece of equipment for any ham. Too often we have a request for the best balun or unun to match a particular antenna. When we ask the customer what the feedpoint impedance is they have no idea. Every antenna is unique and affected by not only the dimensions of the antenna, but also surrounding environment (large structures, metal of all sorts, roof lines, etc) soil conductivity which is different by geography, proximity of other antennas and the list goes on. Without taking an actual measurement with an antenna analzyer at or near the feedpoint, you will have a poor representation of what is going on with your antenna. Notice I say at or near the feedpoint of the antenna. The impact/effect that 100+ feet of coax can make on test results can be substantial when an analyzer is used at the shack end of your feedline.

KK5R and K3KIC like this.

When I trim a dipole to resonance, I start by measuring where the lowest SWR exists, then compute how much you need to trim.
ie if antenna is resonant at 3525 kHz and you want to move it to 3700 kHz
length (feet) =468/ frequency
468/3.525 = 132.77 ft
468/3.700 = 126.49 ft
so trim by (132.77-126.49) ft = 6.28 ft

Your actual length will depend on where your antenna is erected, how close to ground, horizontal or inverted vee, etc
But the amout to trim will come close to what you calculated.

SWR is not necessarily a bad thing, a non-resonant antenna is also no a bad thing.
Many of us use an inverted vee or dipole fed with ladder line very successfully on all hf bands.

73,
Larry KB0R

VK6ATS, KK5R, VK6APZ and 1 other person like this.

The best thing since sliced bread! Do be careful about static electricity from the antenna screwing up the analyzer. And some people have reported strange results if you are close to a commercial broadcast station.

KK5R likes this.
9. VE7KKQHam MemberQRZ Page

Never ceases to amaze me how factual articles like this are missed by the folks with \$5000 radio, a \$2500 amplifier and a \$1500 antenna coupler (tuner) run all this expensive gear into a home brew G5RV at 10 feet high with 200 feet of 25 cents per foot coax.

SA6CKE, KK5R, VK6ATS and 1 other person like this.