Adjustments for resonance

[freecell]

here is anything but a typical 1/4 wl. (ground mounted) vertical antenna.

over average soil: R=100, K=12 / X=0 / 2.7600 meters / 9.0051 feet / 0.251 wl. /
27.192 mhz. / 39.2 ohms load impedance / 93.19% antenna radiation efficiency / 1.275:1 vswr. Rrad=36.8 ohms, Rloss=2.4 ohms

hare's a question for all of you who have been infected with vswrmania. let's begin with a situation that is far more typical than most are aware of. you see a 1:1 swr when measuring a physically short inductively loaded antenna. it must be 50 ohms, right? what if the actual radiation resistance of the antenna is only 25 ohms and there is also an equal amount of loss resistance, also 25 ohms?

let me tell you. given 100 watts of input to the line, under these conditions, half of that power is being delivered to the antenna and the other half is being dissipated in the system loss resistance, such as it is. who would ever suspect such a chased after specification (swr) could leave you with half the power that was put into the line?

what do you know about your antenna network?

 

[338_MtRushmore]

Since we have the internet, it is extremely easy for anyone to figure out what the characteristics of any given antenna should be. If you put a quarter wave on the mobile and show 1:1, it is obvious there is a problem. You can't just completely stop thinking.

Isn't there a section called "low swr for the wrong reasons" or something similar? This is all addressed in a book that you are quoting...

 

[bob85]

My take on this problem is,

The low radiation resistance is caused by the antenna been electrically short which also contributes to the high loss resistance,

a lossy loading coil could also be contributing to the high loss resistance,

25ohm radiation resistance + 25ohm total loss resistance gives you the 1:1 vswr & shitty efficiency.

 

[338_MtRushmore]

X is not irrelevant if that's what you believe the argument is for tuning to vswr dip. Every decently installed antenna I have ever analyzed showed the X curve highly correlated to vswr curve. If they are showing 2 completely different stories, you should suspect a problem.

Given that they both correlate, I'm then going to ignore X if read through a feed line because I know that vswr proves resonance, while X is certain to be skewed by feedline reactance.

 

[Shadetree Mechanic]

If you have an antenna with high swr and you start cutting it, you will end up with a good swr. But maybe it needed to be longer, would have done the same thing. But now the antenna was cut and looks ok. Keep cutting it and it will look bad. Then cut it some more and it will look good again. But will be far away from the frequency you want. This is where the antennalyzer would keep you at your resonant frequency. Then fix the swr with a matching network.

 

[Handy Andy]

Did anybody mention Field Strength?

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Those "extra features" on some SWR meters - they are kinda KEWL - you could use them for many things, location selection for one, but also to see how RESONANT the antenna is, or how well it couples to the vehicles skin and work it as it's counterpoise.

 

[The DB]

Damn, lots been said while I was at work today... Most has been addressed so I will talk about the more relevant parts to me.

A better Field Strength Meter plus an AA look good to me. Sort of like the numbers obtained in baseline performance car owners never obtain. Diagnosis later as time passes makes better tools worthwhile, IMO, as all sorts of mysterious or confusing faults can start to show up.

DB mentiond earlier that a good field strength meter can trump either of the other 2 methods for tuning your antenna, That's true,

Did anybody mention Field Strength?

(picture removed)

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Those "extra features" on some SWR meters - they are kinda KEWL - you could use them for many things, location selection for one, but also to see how RESONANT the antenna is, or how well it couples to the vehicles skin and work it as it's counterpoise.

I've been talking about this on the forum for years now, this thread is the first time I recall anyone else mentioning it... I should also add nomadradio's post from before these...

EDIT: Thinking back I have seen bob85 mention it a few times recently.

With the coax canceling abilities of my VNA's (yes, I have two), I have noticed the SWR and resonance on an antenna, at the feed point, is not necessarily the same point. Modeling has also shown this. However, they will always be be close to each other. An antenna with a wider SWR bandwidth will give more variance than an antenna with a narrower bandwidth when it comes to how close resonance is to SWR.

Peak gain, as measured by field strength, tends to be at a higher frequency then the low (V)SWR and resonant points of the antenna. This is something I see consistently. There are two things that I have found that affect how far away this peak field strength point can be...

1. How narrow the (V)SWR bandwidth of the antenna is
2. How much loss is in the feed line between the radio/amp and the antenna.

Both of these appear to push the peak gain of an antenna system towards the low SWR point. When it comes to feed line losses, by combining modeling and a feed line loss calculator (I like qst's) I can actually show this very thing happening when it comes to an antenna's efficiency/gain.

My take on this problem is,

The low radiation resistance is caused by the antenna been electrically short which also contributes to the high loss resistance,

a lossy loading coil could also be contributing to the high loss resistance,

25ohm radiation resistance + 25ohm total loss resistance gives you the 1:1 vswr & shitty efficiency.

This is absolutely correct. I can give two examples, both based on real world testing and measurements, that show this happening. In one of the cases you increase the antenna's radiation resistance part of R, and in the other you lower the loss component of R, in both cases, this will increase the antennas field strength (or gain if it is done in modeling). That being said, like field strength, when I mention this most people don't seem to care, or even bother to try and understand, so I don't bring it up often...


The DB

 

[HomerBB]

I have noticed on my manual tuner that the peak forward power meter lights are usually showing greater output when the SWR indicator cross needles are not at the lowest SWR, close, but not the closest. Anything relevant.

 

[freecell]

bob85 posted:

"Nick put his analyzer at the radio end of the coax then set about adjusting the antenna for x=0,"

from the users manual:

"a basic understanding of transmission line and antenna behavior and terminology is very important in understanding information provided by the MFJ-269."

c'est la vie.

 

[Slowmover]

I have noticed on my manual tuner that the peak forward power meter lights are usually showing greater output when the SWR indicator cross needles are not at the lowest SWR, close, but not the closest. Anything relevant.

View attachment 44533

My experience also. Except perhaps that being mobile I always opted for dead-lowest SWR on the cross-needle.

.

 

[freecell]

yeah, i would re-think that.

 

[338_MtRushmore]

If you tuned a typical antenna system for x, then swr dip, then field strength, would any receiving station notice the difference?

Obviously it is a hobby and we tend to strive for perfection, but does it really matter? Am I going to get 20% more power if I tune by field strength? Certainly a valve and bipolar wouldn't have the same gain at their optimum impedance?

 

[Slowmover]

yeah, i would re-think that.

As it wasn’t below 1.4:1 I found it adequate to the purpose. Big trucks ain’t easy.

 

[freecell]

yeah, try installing a phased pair of antennas (wilson 2X4' fgt silverloads) on a 1987 (fiberglass) cabover freightliner only to find that with a standard 75 ohm 1/4 wl. phasing harness the system measures a 3:1 vswr across the entire band, no matter how you adjust the tunable tips or whether the links are connected or or not.

 

[Slowmover]

Was able to avoid cabovers way back when, but the dozen big trucks since then I’ve been able to have much better than average CB performance when sweat-equity was applied.

That every Mommy-van out there is better kinda beside the point. Hugely ironic, though.