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Author Topic: Anyone Using Sophisticated SWL Antennas?  (Read 39534 times)
EI4GMB
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« Reply #30 on: May 09, 2012, 03:27:58 PM »

Jeff, we are wasting our time.

You are also wasting the time of others. Learn the theory first guys before you do the practicals !  Wink

73,

Fred EI4GMB
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Edmund Burke (1729-1797)
W0BTU
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« Reply #31 on: May 11, 2012, 04:20:58 PM »

Quote
I have been a BCB Dx'er for many years and my antenna of choice because of limited space is an omni-directional vertical- the RF Systems DX-1 Pro MK II. IMHO it gives comparable results to a Beverage.

Is this the antenna you're talking about that you claim is as good as any Beverage on the BCB?: http://www.universal-radio.com/catalog/sw_ant/1246.html

Oh, BTW, is that a resonant antenna?

And what BCB (that is, frequency range) are you referring to?

I just want to make sure we're on the same page.
« Last Edit: May 11, 2012, 06:10:00 PM by W0BTU » Logged

EI4GMB
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« Reply #32 on: May 12, 2012, 06:40:39 AM »

I know serious DX'ers who have achieved similar results to average sized beverages using the DX-1 Pro antenna.
Their logs are a testament to this.
If you look back on this thread you will see that I have already given an explanation about how this antenna works.
If you have any further questions please feel free to ask.
BTW, I also missed one of your previous edits. A 580 foot beverage would be a good antenna on MW provided it was also a good height above the ground.
It would have a big capture area for receiving signals. However, with a 30 MHz band-width its signal to noise ratio could be improved upon.

Fred EI4GMB

« Last Edit: May 12, 2012, 06:59:17 AM by EI4GMB » Logged

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Edmund Burke (1729-1797)
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« Reply #33 on: May 12, 2012, 09:10:02 AM »

I use an active rotatable Flag antenna and a 7' rotatable active loop. Neither come anywhere close to my 4 Beverages.

Could you explain why a 30MHz bandwidth of a Beverage (of course, it does not function as  a Beverage over 30MHz) yields a poor S/N  ratio. My engineering background in noise theory tells me the S/N of a received signal is totally dependent upon the bandwidth of the receiver channel.
And the narrow front lobe of a Beverage is far superior to other antennas in rejecting noise- see W8JI on RDF- receive directivity factor.

Dale W4OP
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EI4GMB
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« Reply #34 on: May 12, 2012, 01:28:33 PM »

Dale,

Optimum S/N is achieved at antenna resonance or minimum band-width. From the theory I've studied the signal to noise difference occurs first at the antenna end before it reaches your radio. In other words the RF noise is dependent on the capture area of your antenna. The filters of your radio only further help in reducing that noise.
Of course the antenna needs to be matched to your radio.
A simple experiment is to twiddle the controls of your ATU and you will see the difference in S/N when your antenna is matched or not.
Of course I am no expert but I have studied antenna theory. As you are an expert in noise theory maybe you could give us a more comprehensive explanation. I stand to be corrected.

Kind Regards

Fred EI4GMB
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Edmund Burke (1729-1797)
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« Reply #35 on: May 12, 2012, 06:31:01 PM »

Hi Fred,

With all due respect, I don't think you have a handle on the math behind either noise/stochastic processes or information theory.
Resonance has nothing to do with S/N. Yes, signals will be stronger- and so will the noise- unless   you can explain to me how a given  antenna "knows" the difference between signals and noise or point me to a professional article that verifies this.

The antenna bandwidth has absolutely nothing to do with the ultimate received signal to noise- that is 100% a function of the front end noise figure (in the case of VHF and up) and the channel bandwidth.
If I were to believe the above then a 3-30MHz log periodic antenna with say 8dbD gain on 14MHz and F/B of 20dB would not receive (or TX) as well as a Yagi designed specifically for 14MHz that had the SAME gain and F/B. Obviously, such is  NOT the case.

As for an omni active electric field probe performing as well as a Beverage, please review ON4UN's text on section on receive antennas or the W8JI site at:
http://www.w8ji.com/receiving.htm

I think you would  be well served to get a text on information theory or review some of the writings of W8JI.

Dale W4OP
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EI4GMB
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« Reply #36 on: May 13, 2012, 06:23:55 AM »


...Resonance has nothing to do with S/N.

...The antenna bandwidth has absolutely nothing to do with the ultimate received signal to noise.

...I think you would  be well served to get a text on information theory or review some of the writings of W8JI.


S/N has everything to do with antenna resonance. At resonance or minimum band-width an antenna captures less noise. This is self evident and doesn't require a handle on math.

I have read Tom W8JI's work and he is indeed an expert in the field. He is also on here from time to time. Perhaps he could clear up this obvious difference of opinion.

Regards

Fred EI4GMB



« Last Edit: May 14, 2012, 03:02:34 AM by EI4GMB » Logged

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Edmund Burke (1729-1797)
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« Reply #37 on: May 13, 2012, 07:37:43 AM »

Resonance has nothing to do with S/N. Yes, signals will be stronger- and so will the noise- unless   you can explain to me how a given  antenna "knows" the difference between signals and noise or point me to a professional article that verifies this.

The antenna bandwidth has absolutely nothing to do with the ultimate received signal to noise- that is 100% a function of the front end noise figure (in the case of VHF and up) and the channel bandwidth. ...
As for an omni active electric field probe performing as well as a Beverage, please review ON4UN's text on section on receive antennas or the W8JI site at:
http://www.w8ji.com/receiving.htm

I think you would  be well served to get a text on information theory or review some of the writings of W8JI.

Very well said, Dale! That's one thing people overlook: as far as an antenna, receiver, etc. is concerned, there is no difference between noise and the desired signals.

When you increase the signal by bringing an antenna or tuner into resonance, the levels of both the signal and noise coming out of our speakers or headphones increase at exactly the same rate. The noise doesn't get "left behind" at a lower level than the desired signal. Their relative levels (the S/N ratio) remain the same.

I know a guy who thinks that he is going to reduce unwanted noise (and therefore increase his S/N ratio) by taking down his ordinary dipole and replacing it with a certain design of of folded dipole, which he claims is a "quieter" antenna. Ain't gonna happen. Oh, there may be differences in the antenna that may reduce corona discharge, or the feedline of one antenna may be less susceptible to noise pickup, but unless he changes the pattern of the antenna, he's wasting his time. But nothing anyone has said to him will convince him otherwise.

As for Tom, I don't believe that he has been on either eHam or QRZ since March. But he speaks of using separate non-resonant antennas for receiving on his site. If resonant antennas helped improve the S/N ratio, I think he would state it.

At http://www.w8ji.com/receiving.htm he very nicely explains how low-noise receiving antennas really work. He states that "... as a general rule, the ratio of peak response in the direction of the signal to average response in all directions is the dominant factor in determining how good an antenna works for receiving. In virtually all installations without clearly dominant direction or directions of noise arrival, RDF (receiving directivity factor) very accurately predicts receiving antenna performance."

Nowhere does he mention resonance helping the S/N ratio, because it doesn't.

BTW, I love those homebrew projects of yours on your web site, Dale!
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EI4GMB
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« Reply #38 on: May 13, 2012, 12:35:36 PM »

For best reception it is necessary to ensure that the performance of the RF antenna design is optimum.

I don't think a beverage antenna fully meets this requirement. How would it? It is not even matched to your radio.

The S/N (dB) is the difference between the noise and signal of the receiver channel when it is connected to a matched source !

I happen to believe that an antenna that is matched on its band of operation is less noisy than an antenna which is not. That's all !

I am not making any wild claims just giving some observations based on my own antenna experience.

73,

Fred EI4GMB
« Last Edit: May 13, 2012, 01:54:33 PM by EI4GMB » Logged

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Edmund Burke (1729-1797)
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« Reply #39 on: May 13, 2012, 02:03:26 PM »

For best reception it is necessary to ensure that the performance of the RF antenna design is optimum.

I don't think a beverage antenna fully meets this requirement. How would it? It is not even matched to your radio.

The S/N (dB) is the difference between the noise and signal of the receiver channel when it is connected to a matched source !

I happen to believe that an antenna that is matched on its band of operation is less noisy than an antenna which is not. That's all !

I am not making any wild claims just giving some observations based on my own antenna experience.

73,

Fred EI4GMB

This will be my last post on the subject as I can see others have posted the same concepts.

First, the Beverage is a travelling wave antenna- resonance is not an issue, nor is it desirable. The Beverage is however matched to the receiver by a transformer. The gain is quite low, thus the matching. One might also note that the active antennas are also nowhere close to being resonant- they are a very high Z electric field probe whose impedance is brought  down via  a source follower or emitter follower.

Matching an antenna to the receiver (at HF) means more signal (and noise in an equal amount) is received. However, efficiency is not an issue at HF. Our noise floor is determined by:
1. Atmospheric noise
2. Galactic noise
3. Manmade noise

If you doubt this, turn your radio on to  any HF frequency with no signals, then disconnect your antenna. If the noise level does NOT drop, then there is something wrong with your receiver.

Isn't it odd that the top receive antennas for 160M are all non-resonant, have very low gain, but do have excellent patterns and therefore high RDF?

Beverages
Multiple/phased  Beverages
EWE's
Flags
Pennants
Receive 4 squares and their ilk- i.e short E field phased verticals

NONE of these antennas are anywhere close to being resonant and yet the world's top  DXers all prefer them Hmmmmm.

Check out K7TJR's site of low band receive antennas. For the uninformed,  RDF is the figure of merit here- the higher the RDF, the better the antenna (i.e. its ability to reject noise from unwanted directions). Note that the WORST of the lot is a resonant inverted Vee at an RDF of 0.6:
http://www.k7tjr.com/rx1comparison.htm

As soon as you find someone to agree with your "resonance is best for RX antennas",  let us know Fred. Tom, W8JI is always available to offer his thoughts.

73,

Dale W4OP
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EI4GMB
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Posts: 166




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« Reply #40 on: May 13, 2012, 03:28:27 PM »

Dale,

You have described the beverage antenna as a travelling wave antenna yet have said nothing about how this title effects performance.

You have also stated that a beverage antenna is matched to your radio via a transformer.  What type? and how does this effect antenna performance?

A receiver channel needs to be connected to a matched source i.e antenna, to determine SNR.

S/N is dependent on many things. I believe matching your antenna to the band of operation does help.

Finally, there would appear to be a lack of expertise around the whole issue of resonance on both sides of this debate.

I am prepared to wait for a more knowledgeable and scientific explanation.

Best Regards

Fred EI4GMB
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Edmund Burke (1729-1797)
EI4GMB
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« Reply #41 on: May 14, 2012, 07:44:21 AM »

Hi Guys,

I have discussed this subject with my good friend, Steve G3TXQ of Hexbeam fame.
He reliably informs me that matching an antenna to a receiver won't have any discernible effect on noise because we are generally externally noise limited at HF. So I stand corrected.
Things are different at VHF/UHF (and sometimes on 10m) were we are typically limited by the front-end noise of the RX, in those situations getting the maximum signal transferred from the antenna to the RX by correctly matching the two is important.
On the other hand, the Beverage is not a 'typical' HF antenna !
By virtue of the way it works close to the ground it is quite lossy; its gain might be as low as -25dB.
So you do have to be careful that it is reasonably well matched to the RX to ensure the S/N ratio is not determined by radio front-end noise.
It's also important that the Beverage is terminated correctly in order for it to act as a transmission line and provide directivity.

Anyway, I just thought I would add this post to the record of the thread.
There seems to be plenty of information here for us all to consider.

73,

Fred EI4GMB
« Last Edit: May 14, 2012, 08:17:51 AM by EI4GMB » Logged

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Edmund Burke (1729-1797)
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« Reply #42 on: May 15, 2012, 06:25:04 AM »

What did Steve say about resonance affecting S/N ratio?

Here's what W8JI said about it: http://www.eham.net/ehamforum/smf/index.php/topic,80266.msg567904.html#msg567904
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W4OP
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« Reply #43 on: May 15, 2012, 10:29:46 AM »

Makes perfect sense. Thanks for the link to Tom.
Again, to think that resonance has anything to do with S/N at HF is to assume that the antenna must "know" the difference between a signal and noise on the channel. To think that an antenna's bandwidth limits sensitivity is also wrong as it is the following stages that determine the ultimate S/N. The only exception I can think of is that if the receiver's IP3 is so poor that the wide range of a broad band antenna might overload the front end. Then again, simple wire antennas are a VERY POOR filter even if narrow band, and modern receivers are not prone to overload.

In addition, Fred has commented that resonance implies  a match (which implies maximum power transfer). As we know, even EWE's, Pennants, and Beverages with their very low gain, still are plenty sensitive for HF. I do amplify my Beverages but only to bring their overall level up to that of my TX antennas, making on-air  comparisons easy.

Back to resonance. Resonance simply means the lack of reactance and has zero to do with a good match or VSWR except for a very small class of antennas.
Imagine, if you will,  a resonant end fed half wave antenna. It's end Z is a function  of conductor diameter and height above ground- but for wire antennas at HF will be somewhere in the 2K--> 4K range. This  is a VSWR of 40:1 up to 80:1.
So it is not only true that a resonant antenna has nothing to do with S/N but also does not guarantee a match or low VSWR.

Aside from dipoles at certain heights, Moxons, closed waveguides with a 1/4 wave probe,  and Half Squares, most antennas are not resonant and/or do not present a 50 Ohm match without requiring  require some sort of matching if they will be used for TX. For RX, it's a non-issue.

Finally, measure the input Z of your HF RX. You will find in most cases, it is anything but 50 Ohms resistive over its spectrum
Dale W4OP
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EI4GMB
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« Reply #44 on: May 15, 2012, 12:22:59 PM »

What did Steve say about resonance affecting S/N ratio?

That resonance wouldn't have any affect on S/N.
As proof of that you could place a 10dB attenuator between the antenna feed and the input of the RX on 40m and you wouldn't change the S/N ratio because both signal and noise would drop by the same 10dB.
I accept the expert opinions of Steve G3TXQ and Tom W8JI and stand corrected.

Best Regards

Fred EI4GMB
« Last Edit: May 16, 2012, 03:05:01 PM by EI4GMB » Logged

'You can never plan the future by the past'

'Those who don't know history are destined to repeat it.'

Edmund Burke (1729-1797)
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