Resonance and the low impedance point.

[The DB]

This is continuing a discussion started in this thread. I moved the discussion here out of respect for the op as we were getting a bit off of the question asked.

This discussion has come about because I have made the comment that a resonant frequency of an antenna will also be a low impedance point. More information is in the thread linked above.


The DB

 

[The DB]

I'm thinking there are examples where tuning for "the dip in impedance" may not put you close to resonance. Take any antenna that uses an inductor at its base with a tap point on it that connects to the feedline. Could be anything from your Wilson 1000 mag mount to your Maco V-58. These matching networks are nothing more than RF versions of the basic auto transformer.

I base my knowledge on the ever growing collection of antenna books I have (new and old) and even more so from my experience with working with antennas. I have done my share of tuning and building in the past. I have several designs in my head that I will hopefully get a chance to build and play with next year that would potentially build on what I already know.

Is it possible I am wrong on this? I cannot say no. I can only put forth what my experience (I'll be it limited compared to some on this forum) has shown me, and that is when you have a resonant point you also have a low impedance point. I have also been careful not to make any specific claims on what that low impedance value may be.

The closer you move the tap point towards the grounded side of the inductor, the lower the impedance will be. You could continue to lower the impedance until you've reached a short. You want the impedance to match the feedline but that doesn't guarantee resonance has been achieved either.

I don't think what you said and what I said are mutually exclusive here. I am not arguing that adjusting the tap point will change how big the dip in impedance is, however there are other changes happening here as well. For example, adjusting the tap also changes the low impedance frequency, and the resonant frequency follows suit. This is why when tuning these antennas you not only adjust the tap but the overall length to achieve the feed line impedance as well as resonance.

NOTE: I have never pushed this setup to its extremes, however, I don't see tuning it to a very high or low impedance changing anything here.

You could take a center fed gamma matched dipole such as the Sirio S-27 and adjust its elements length so that it was no longer resonant. At the same time you could adjust that gamma match to show a wide range of impedance including 50 ohms without resonance.

I am well aware that it is possible by any number of means to tune the impedance of a non-resonant frequency to a wide range of impedances. In theory you can match the impedance of (insert any conductive item here) to any feed line impedance with some form of tuner/matching unit at any frequency. I fail to see what this has to do with what I said. Can I get more information on how this relates?


The DB

 

[Shockwave]

Many antennas deal with resonance and impedance matching as totally separate areas. Where the length of the radiator will determine resonance and the tap on the transformer or gamma controls the impedance.

This is the case with your 1/2 wave end fed groundplane and the 5/8 wave groundplane. When adjusting the tap point on these antennas, there is no dip to be found. You see a change from low impedance to high impedance as you move from the grounded side to the radiator side of the inductor. These antennas use the transformers to match impedance, not resonance because they do not function as loading inductors.

The exceptions here would be the base loaded and matched Wilson 1000. Because the coil here is part of a loading and matching network, its tap point will have a big affect on impedance and resonance. However, the impedance will still continue to fall as the tap is moved closer to ground without a dip.

The reference to the S-27 dipole was just another example of how resonance and impedance can function independently from one another and that tuning for the lowest impedance does not guarantee the radiator is resonant. I'm not suggesting there aren't any cases where resonance will line up with a dip in impedance.

In fact many old HF marine radios would use a #47 pilot lamp in series with the antenna as a tuning indicator for the 1/4 wave end fed antenna. When the light was brightest, the antenna was tuned. My point is there are plenty of antennas with matching networks that can be tuned without finding a dip in impedance that present a wide range of impedance adjustment with little affect on resonance.

 

[wavrider]

DB exactly as shockwave said.

Resonance and impedance are two different things.

Resonance is determined by wave length or frequency, one and the same maybe.

First cup of coffee here so hope some of what i post makes sense.

Example.
Two element Quad Any freq it is around 100 ohms impedance .
Length of radiator is cut to length dependant on freq you want. Impedance is not even considered in length of radiating element.

Once it is cut and built then an impedance matching device is needed to bring it down to 50 ohms.

 

[The DB]

Sorry for delay, was out of town, hoped to get a reply before I left actually...

First off, to Waverider, I know they are separate, old news buddy. I have said that more than once on these very forums. I was arguing that there was some relationship between the two, and can demonstrate this relationship with several different antennas. Can I write a mathematical proof that proves it, no. I can say it is an effect that I have seen enough to be able to establish a correlation. Is it possible that I have made this correlation in error? It was based on inductive reasoning, so admittedly there is always that chance. I am not set on any piece of knowledge as long as I can be shown ample evidence that I am wrong and cannot contradict it with my own experiences.

To Shockwave, an observation and a question.

Observation: On a Maco style tuning system, if you move that tap point enough the resonant point does change. I know you have said otherwise, but I have personally seen this more than once.

Question: If this matching network does not effect resonance how then does the non-resonant shorter than 5/8 wave antenna appear as resonant through an antenna analyzer?


The DB

 

[W5LZ]

Try thinking of it like this.
Resonance is the absence of reactance, or the reactance is 'nulled'.
The 5/8 wave length at any frequency is not resonant. It has to be made resonant by adding the appropriate reactance. That adding is done in that 'matching' device at the base. That matching device is composed of two parts, the impedance matching part of the coil, and the part of the coil assembly that adds that required reactance to that 5/8 wave length to make it resonant. That coil assembly is two things not just one. The part where the center conductor of the feed line attaches and below determines the input impedance of the antenna. The part of that assembly above that attachment point is a loading coil that provides the required reactance to make the thing resonant.
That help any?
- 'Doc

 

[The DB]

Try thinking of it like this.
Resonance is the absence of reactance, or the reactance is 'nulled'.
The 5/8 wave length at any frequency is not resonant. It has to be made resonant by adding the appropriate reactance. That adding is done in that 'matching' device at the base. That matching device is composed of two parts, the impedance matching part of the coil, and the part of the coil assembly that adds that required reactance to that 5/8 wave length to make it resonant. That coil assembly is two things not just one. The part where the center conductor of the feed line attaches and below determines the input impedance of the antenna. The part of that assembly above that attachment point is a loading coil that provides the required reactance to make the thing resonant.
That help any?
- 'Doc

The first line I knew. I fully understand resonance and reactance.

As for the rest of it, I had a thought just after I posted the message along those lines. The only hiccup I see is I have tuned these antennas and on multiple occasions changing the tap point changes the resonance shown on an analyzer plugged directly into the antenna as well, as well as the low impedance frequency. In these cases I moved nothing but the tap point.

I am going to examine my point of view on this. I know its not completely false as I can demonstrate the effect, but there are designs that I have little to no experience with. I guess at this point I need more experience with different antenna designs... Unfortunately again, I have to wait on that...


The DB

Don't mind me while I figure out what I don't know so I can determine what path to take to learn more...

 

[Shockwave]

Sorry for delay, was out of town, hoped to get a reply before I left actually...

First off, to Waverider, I know they are separate, old news buddy. I have said that more than once on these very forums. I was arguing that there was some relationship between the two, and can demonstrate this relationship with several different antennas. Can I write a mathematical proof that proves it, no. I can say it is an effect that I have seen enough to be able to establish a correlation. Is it possible that I have made this correlation in error? It was based on inductive reasoning, so admittedly there is always that chance. I am not set on any piece of knowledge as long as I can be shown ample evidence that I am wrong and cannot contradict it with my own experiences.

To Shockwave, an observation and a question.

Observation: On a Maco style tuning system, if you move that tap point enough the resonant point does change. I know you have said otherwise, but I have personally seen this more than once.The DB

On the Maco the match is set to cancel reactance at the tuned frequency. It is when the inductive reactance and capacitive reactance are of the same magnitude that they cancel and provide us with the resistive load to match the transmission line.

Changing the tap point or capacitor setting on a gamma will not change the electrical wavelength or resonant frequency of the radiator. It will simply give you a different value of reactance that can now provide the match at a frequency where the radiator is not resonant. This can reduce gain depending on the particular antenna design because the element is not resonant and the match is correcting a higher degree of reactance.

This is also why you do not adjust the radiator length on antennas with adjustable matching networks to provide a good VSWR. The element is adjusted so it's resonant at the desired frequency and the matching network is adjusted to provide the resistive match to the coax.

Question: If this matching network does not effect resonance how then does the non-resonant shorter than 5/8 wave antenna appear as resonant through an antenna analyzer? The DB

The answer here is very similar to the first explanation. Once a matching network is added, you no longer need a resonant radiator to transform the drive impedance back to 50 ohms. Think of the matching network as an antenna tuner. You could match anything from 4 feet to 400 feet with a decent tuner. It's just a matter of adding the right type and magnitude of reactance into the system that will cancel those within the specific radiator being used.

 

[Shockwave]

Question: If this matching network does not effect resonance how then does the non-resonant shorter than 5/8 wave antenna appear as resonant through an antenna analyzer?


The DB

To follow up on this, in order to match a 5/8 wave that is electrically short of 5/8 wavelength, you'll need some capacitive reactance in parallel with the antenna to ground. The Maco 5/8 wave has this very cap hidden in its design. The base insulator is formed from several inches of overlapping aluminum tubing separated by a dielectric insulator. That is the cap and what fails when you hit it with too many watts.

 

[Shockwave]

Here is an interesting experiment. Let's take the Wilson 1000 base loaded 1/4 wave as an example. Most of us know that the full 1/4 wave would be closer to 108 inches and that the coil in the base is compensating for the shorter whip. Some even know if they cut too much off the whip, they could open the coil and add another turn to the top of the coil to compensate for an even shorter whip.

Common sense would suggest the same thing should apply to the 5/8 wave groundplane or a 2 meter 5/8 wave mag mount. Try retuning either of these antennas by just shortening the radiator length so that the VSWR is flat approximately 2 MHz. higher than the desired frequency. Then do whatever you want to the coil at the base and you'll never get a flat VSWR at the desired frequency.

One might think all you need to do is add a little more inductance or loop above the tap point to load the radiator. This experiment will advance your learning by demonstrating the inductor at the base of a 5/8 wave cannot function as a loading coil or alter the resonant frequency as it does in the loaded 1/4 wave. It is strictly a matching network that can only add inductive reactance and the shorter 5/8 wave end fed needs capacitive reactance to be added into the match.

 

[The DB]

Thanks for the responses Shockwave.

Looking at the design it makes sense that there is capacitance being introduced. The tubes are clearly one inside the other with an insulating material between them. I can't believe I didn't see that before as I have made some capacitors in the past (including a variable capacitor using two copper pipes sliding one inside the other with a plastic scrubby used to wash dishes as an insulator between them). Now that part of the design reminds me of a gamma (which I admittedly have very little experience with), although in this case there is obviously more to it.

Anyway, as I said before, I have some experimenting on the topic to do.


The DB

 

[IRock817]

While this is still a warm thread! How does the gamma system on a pdl work. I have built a four element and am having difficulty tuning for lowest swr. Do I need to slide the center rod of the gamma tube or just move the connector on the (what I call) the globe?
This may save me a lot of time and anguish!

Thanks for any input!

 

[Shockwave]

While this is still a warm thread! How does the gamma system on a pdl work. I have built a four element and am having difficulty tuning for lowest swr. Do I need to slide the center rod of the gamma tube or just move the connector on the (what I call) the globe?
This may save me a lot of time and anguish!

Thanks for any input!

You have to adjust both parts. Sliding the rod is one adjustment and the connector is the second. Hopefully you didn't buy those heavy duty 5 KW PDL gammas on eBay because the fiberglass insulation they used changed the tuning drastically. Also consider the impedance of a 2 element quad is going to be quite a bit higher than a 4 element so expect to move the settings away from the stock positions.

 

[wavrider]

While this is still a warm thread! How does the gamma system on a pdl work. I have built a four element and am having difficulty tuning for lowest swr. Do I need to slide the center rod of the gamma tube or just move the connector on the (what I call) the globe?
This may save me a lot of time and anguish!

Thanks for any input!

With a four element quad the input impedance is roughly 50 ohms.

Since you are using a PDL type driven element, then the gamma will need adjusting.

Slide the center gamma rod in or out for lowest VSWR reading.

Once you have reached lowest vswr by moving center rod in or out then loosen the connector on the "Globe" and slide it up or down.

Some where between the two you will find a 1:1 VSWR match

 

[IRock817]

Thanks fellas! We did build new gamma's from tethlon! These style gammas have been in use for years.
I'll lay it down and face it in the air, and tune per your instructions. When we tuned it last, we just moved the gamma on the globe and couldn't seem to get it right. Knowing to adjust the rod as well is a relief.
I'm looking forward to seeing if getting it tuned actually helps. It works great now, anything better is a bonus.

Thanks again.