The Antenna Dipper.

Started by gil, September 04, 2012, 12:07:43 am

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gil

A well tuned antenna will make your radio hear better and reach further. It will also avoid damage to the final transistor(s) that amplify the signal to the antenna. That is why you can't plug in any length of wire to a transmitter without some kind of tuner. It is always best though to operate without a tuner if you can. Antenna length depends on frequency. The higher the frequency, the shorter the antenna, and vice-versa. The usual method of making an antenna is to leave it a little longer than required, and measure how much power is radiated versus how much power returns to the transmitter. Ideally, all the power generated should be radiated. The measuring instrument that does this is the SWR meter (Standing Wave Ratio). You plug it in between the antenna and the radio.

The problem with the SWR meter is that you actually have to use your radio and transmit a signal to find out the correct antenna length (by trimming it inch-by-inch). You can only transmit a couple seconds at a time (in case the SWR is dangerously high), and at reduced power if possible. So how do you tune your antenna if you don't have a license yet or don't want to draw attention by broadcasting while tuning? Here comes the antenna dipper!

The best way would be to use an antenna analyzer, which is a combination of radio frequency generator and SWR meter. Some models do much more, like displaying a graph. These gizmos however start at around $250. As much as some good radios! The advantage of the antenna analyzer as well as the dipper is that the signal used to measure SWR is very weak. Not much gets out.

The antenna dipper is much cheaper and works quite well. You just don't get a fancy display of your SWR. All you have is a frequency display and LED to show where your antenna is tuned.






The one I built is the "Deluxe Tenna Dipper" from http://qrpkits.com. At $85 it is much cheaper than an analyzer. It was easy to build and works quite well. You plug it in your antenna wire and turn the frequency dial until the light goes out. The frequency then displayed is your resonant frequency. That's it! If the displayed frequency is higher than you want, you need to lengthen the antenna, if is is lower, you shorten it... Nothing else to it. It goes up to about 30MHz.

The Hendricks kits are quite good (I am not associated with them), and work very well. I have built the SOTA tuner, SWR indicator and the DC20B CW transceiver. They all worked just fine. I highly recommend the SOTA tuner for low power operations (max 5W)!

I had some safety-orange spray paint left over, and it worked out well.

So, before you spend $300 in an antenna analyzer, or risk frying your new radio's finals by trying to tune you wife's clothesline, think about the simple antenna dipper. While not necessary (A $30 SWR meter works fine), it does save the experimenter a lot of time. You can leave the dipper on and look at the LED while at the other end trimming wire. No need to involve your eye-rolling significant other  ::) The LED is quite bright. If you want to tune an antenna for best reception without transmitting a signal, it is, with an analyzer, the only way to go. I found out that my CB antenna (Solarcon I-MAX2000) not only works for CB (11m), but also 10m and 15m! 10m I suspected would work, but I had no idea 15m would. Surprisingly, 12m does not..

I love building antennas and this little guy is great!

It doesn't mean I won't end-up buying an antenna analyzer (Ten-Tec sells a neat el-cheapo Chinese model with color screen), but right now money is tight, and the "Tenna Dipper" does it's job...

Paul

Got some good news and bad news for you.  Your 'dipper' will work as well as any SWR meter will at 'tuning' an antenna.  That's the good news.  The bad news is that an SWR meter can't tell you anything about resonance, and neither can your 'dipper'.
The only thing an SWR meter can do at the very best of times is indicate when the input impedance of what's on the 'back' side of it is at least near 50 ohms, which is what should be on the 'front' end of it.  That includes the feed line in that impedance matching NOT just the antenna.
A 50 ohm input impedance does not mean resonance.  Resonance may not be (very seldom ever is) 50 ohms impedance.  That means that a resonant 1/4 wave antenna will not have an SWR of 1:1 since it's 'normal' input impedance is something like 25 - 35 ohms.
Tuning an antenna always means two things, making that antenna resonant and matching it's input impedance to the rest of the systems impedance.
- Paul


gil

Correct! A perfect example is the dummy load.. The trick here is to use an antenna designed to present a 50ohm impedance at resonance. That isn't often the case however. A regular dipole will present an impedance of about 73ohm at resonance. You can lower the ends a bit and bingo, 50ohm at resonance. Of course, you changed the radiation pattern doing that.

A SWR of 1:1 isn't always best, and perfect resonance isn't always best either. What matters is how much RF is radiated by the antenna. Some non-resonant antennas will radiate more than a resonant one. A field strength meter tells the story..

My take on all this is to make the antenna as near-resonant as possible, and match impedance at the feed point when possible to reduce line losses. Books have been written on the subject, and I certainly don't know it all, so I won't elaborate. I have a lot to learn..

Aren't antennas are a lot of fun?  :)

What the dipper does is tell you that your radio won't be damaged by reverse power, and that's it. It also speeds up antenna adjustments.

Gil.

Paul

Actually, lowering the ends of a 'flat' dipole to lower it's input impedance isn't going to affect the antenna's radiation pattern all that much.  Some, sure, but nothing drastic really.  There are other things that will have a much larger affect on radiation patterns than that 'angle of dangle', height above ground for instance.  Or things near that antenna, trees, buildings, whatever.  All of those things affects the radiation pattern, you'll almost never 'see' a classic pattern, it'll always be 'distorted' in some way.
- Paul