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Author Topic: Active vs. Passive Antennas for HF SWL  (Read 18524 times)
W8BYA
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« on: March 09, 2013, 11:27:21 AM »

Hello group.  I do not own nor have had the opportunity to play with any active omnidirectional HF SWL antennas.  My experience has always been with various random & monoband outdoor wire antennas, and yagi's cut for the various ham bands.

Nowadays, through modelling, I know that these antennas can start to have complex & very undesirable azimuth & elevation patterns once removed from their intended operating frequency.  Having an outdoor omnidirectional, yet sensitive active antenna seems like it would make a nice addition to the copper I currently use.  Has anyone here had an opportunity to try one against the other and be willing to share their experiences?  Or maybe someone can point me to a URL where I might find such information.  Thanks !

73 Gedas, W8BYA 
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K0OD
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« Reply #1 on: March 09, 2013, 11:11:35 PM »

"Many people have misconceptions about active antennas. Some think that their radio receiver system will be transformed into a highly sensitive “spy grade” radio that will hear everything that is broadcast around the world. This is simply not so."
http://wiki.radioreference.com/index.php/Active_Antennas_KB6RWX

Clifton Labs: Mostly about active small loops:
http://www.highgainstore.citymax.com/f/Jack_Smith_Review1.pdf

Clifton Labs: Improved active antenna installation (for VLF but may help HF too). 
http://www.cliftonlaboratories.com/vlf_signals_with_improved_setup.htm
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W8BYA
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« Reply #2 on: March 10, 2013, 01:54:29 PM »

The work and level detail by K8ZOA is most impressive.  After Jack provided a brief description of the available antennas he has at his disposal, I was drooling & hoping for some comparisons between all of them.  I looked for additional articles on his site but could find any. Now that would be interesting since many of us do have V's and dipoles and yagis for bands other than the SWL bands.  Do you by chance know of any such reports ?

Ref comments by KB6RWX, I think a more interesting part of his report was:
"Active antennas will generally not perform better than a full size (resonant) antenna or even a long wire. Even so, active antennas are useful to those desiring to achieve adequate performance, but do not have the space for a long wire or other full sized antenna. Another major advantage is that is that an active whip is omni-directional which a long wire is definitely not. In fact, long wire antennas are fundamentally bi-directional and can be extremely directional depending on its physical length with respect to the signal frequency involved."

This makes me even more interested since if the small active antenna can even get close to the performance of a full-sized or resonant dipole antenna for one of the SWL bands, then IMO it may be very usefull.

Gedas, W8BYA
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K0OD
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« Reply #3 on: March 10, 2013, 04:30:40 PM »

Quote
"This makes me even more interested since if the small active antenna can even get close to the performance of a full-sized or resonant dipole antenna for one of the SWL bands,"
But resonance in a receiving antenna doesn't help its ability to pull in weak signals at least on hf bands and lower where noise is the enemy, not weak signal strength.

This topic interests me. I gave thought to using a loop--probably an amplified or "active" one-- to improve my longwave reception.  I doubt that I'd benefit on HF from such an antenna. Might not even benefit on longwave. My ultra non-resonant HF ham vertical actually hears well down to 10 kilohertz where it's less than one percent of a quarterwave. 

Advantages of active antennas:
1) where conventional antenna is impossible... inside an apartment, hotel room etc.
2) to move the antenna away from local noise sources... lights, computers, power lines, etc That's why embassies use them.

Additional advantage of an active loop:
3) nulls on ends sometimes are useful.
4) for a bunch of SWL types, makes house look like a CIA station
http://www.angelfire.com/rebellion2/sw_underground1/embassy/czechloops.jpg


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W8BYA
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« Reply #4 on: March 10, 2013, 05:07:25 PM »

Had to chuckle...."4) for a bunch of SWL types, makes house look like a CIA station
http://www.angelfire.com/rebellion2/sw_underground1/embassy/czechloops.jpg" Too funny.

But, like you, this is an area of interest for me.  I should have removed the word "resonant" from my text....I actually know better....I only included it as that was a direct quote from KB6RWX.  I too have non-resonant antennas, designed that way on purpose by me that work very well.  I am actually leaning towards a small active vertical antenna for my initial HF scanning around.  I have room outside and live in a quiet environment and could put up a loop or vertical.  Altough I must say, like this evening, I was floored at how seemingly well even my R8 worked as I tuned around on the various SWL bands.

The thing that is most intriguing for me is to be able maintain a nearly constant azimuth and elevation pattern across a very wide range of HF frequencies.  As long as I have the S/N I will be happy.  Just trying to get a feel from others before looking further or before home brewing something myself.  TU very much for the neat links earlier and thoughts most recently.

Gedas, W8BYA
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K0OD
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« Reply #5 on: March 10, 2013, 07:51:14 PM »

Jack Smith, K8ZOA and his Clifton Laboratories company have a good reputation. Notice that his study of active antennas is just Part One:
Active Loops, Active Voltage Probe and Wire Antennas; A Statistical Review of the Performance of Six Antennas (Part I)- January 2012
http://www.highgainstore.citymax.com/f/Jack_Smith_Review1.pdf
I'm pretty sure Part Two isn't out yet.

He gives us in part I 18 pages about the Welbook ALA 1530L, Pixel Pro-1b, Clifton Z150F, and others. He also includes:

• Antenna pattern null depth
• Mechanical design and mounting
• Maintainability and customer support
• Intermodulation distortion in strong signal environments
• Compatibility with Transmitters
• Connectors and coaxial cable requirements

What a wealth of otherwise unobtainable info for buyers and builders of active antennas. I presume Part Two will be out soon. His web site, which is pretty disorganized, is http://www.cliftonlaboratories.com.


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K1DA
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« Reply #6 on: March 19, 2013, 10:01:06 AM »

No question,  "resonant" is not what one should be chasing as the Holy Grail, , but making sure a receiver sees 50 ohms at the 50 input terminal IS important for signal strength purposes.  Not many "high end" receivers come with an HF "long wire" input these days, if they do it is for use below 500KHZ or so.  A used "low power" tuner with a long wire input, like the dreaded MFJ series  is fine. 
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W4OP
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« Reply #7 on: March 19, 2013, 10:42:31 AM »

No question,  "resonant" is not what one should be chasing as the Holy Grail, , but making sure a receiver sees 50 ohms at the 50 input terminal IS important for signal strength purposes.  Not many "high end" receivers come with an HF "long wire" input these days, if they do it is for use below 500KHZ or so.  A used "low power" tuner with a long wire input, like the dreaded MFJ series  is fine. 

I am not sure why you say  a 50 Ohm Z for the antenna is  important. If there is a mismatch, what is the problem? Signal and noise will perhaps be less, but in equal measure- so the resultant signal to noise ratio is unchanged.

Dale W4OP
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K0OD
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« Reply #8 on: March 19, 2013, 11:04:27 AM »

Receiving tuners may be important for "signal strength purposes" and "to make the receiver happy" [whatever that means] and they're certainly important to people in the business of marketing... tuners.  But the purpose of a receiver is to receive. A tuner will almost never let you receive (hear or copy) a weak signal better. Detune your tuner or switch it out of the signal path and see if weak stations disappear. They won't. Save your money and reduce shack clutter.

Many of the best receiving antennas, such as beverages, are terrible measured by "signal strength purposes."

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K5TED
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« Reply #9 on: March 19, 2013, 05:30:42 PM »

An active tunable magnetic loop antenna will most often give better S/N than an equivalent sized active vertical whip.

A good preselector will often serve to greatly reduce interference.

A typical inexpensive tuner like the MFJ 945E will do wonders to improve the usability of a long wire, acting as a preselector.

Another option is to find an old McKay Dymek, Stoner/Dymek or Palomar.

It's also a very simple project to throw together a multi-turn 5' or 6' wire loop with a tuning cap. YOu might be surprised at how much difference it will make compared to a simple random or long wire.

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W4OP
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« Reply #10 on: March 19, 2013, 05:38:32 PM »

"A good preselector will often serve to greatly reduce interference."

Unless the RX front end is being driven non-linear, how does a preselector reduce interference?

Dale W4OP
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K5TED
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« Reply #11 on: March 19, 2013, 06:04:49 PM »

It rejects unwanted out of band signals.

Try it.

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W4OP
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« Reply #12 on: March 20, 2013, 07:03:34 AM »

That's what the front end BPF or LPF's are for. So again, unless the front end is being driven into IMD, the receiver doesn't care. Now admittedly, I have never used the Etons, Grundigs etc, but even my Lowe HF-150 that has no passive front end filtering does not have a problem with full sized dipoles and Yagis. My home brew receivers have an IP3 of +7dBmV and +11dBmV (as measured in my lab) and  never had an issue.
http://www.parelectronics.com/par-homebrew-projects.php

Photos 1 and 2


Dale W4OP
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K1DA
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« Reply #13 on: April 03, 2013, 08:10:53 AM »

Not every person getting into shortwave has, or can afford a high end receiver at the onset.  Many a reasonably priced "general coverage" radio front end could use a little help.  Going on about what you built yourself and how good in may be doesn't bring many beginners into the hobby.
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K1DA
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« Reply #14 on: April 03, 2013, 08:12:45 AM »

No question,  "resonant" is not what one should be chasing as the Holy Grail, , but making sure a receiver sees 50 ohms at the 50 input terminal IS important for signal strength purposes.  Not many "high end" receivers come with an HF "long wire" input these days, if they do it is for use below 500KHZ or so.  A used "low power" tuner with a long wire input, like the dreaded MFJ series  is fine. 

I am not sure why you say  a 50 Ohm Z for the antenna is  important. If there is a mismatch, what is the problem? Signal and noise will perhaps be less, but in equal measure- so the resultant signal to noise ratio is unchanged.

Dale W4OP
  Maybe if you read my post you would realize I referenced RECEIVER INPUT impedance matching as important, NOT antenna Z. 
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