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Author Topic: Anyone Using Sophisticated SWL Antennas?  (Read 39527 times)
K0OD
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Posts: 2521




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« on: March 17, 2012, 08:08:54 AM »

For most SWLs an ever-so-simple random end-feed wire is quite adequate. I have used my ham vertical (basically a random wire on most frequencies) to pick up European longwave stations as low as 150 Hz, as well as shortwave broadcast stations on HF and even VHF.

But nothing beats a directional antenna for gain and QRM/noise reduction.  No doubt wideband log periodic yagis work. You'll see them on government and commercials buildings. But most us don't want to put up something as big as a Volkswagen high over our homes. And even log periodics are compromises. This "small" one has a low frequency cut off of 6.5 MHz and its performance within its rated range is dependent on its height above ground.
http://www.antenna.it/military/images%20military/image.c.630-lp10.jpg

More practical small, easily erected, moderate price SWL antennas are available. Loops have been around from the start of radio and sometimes offer excellent noise and QRM reducing nulls. Also available are simple active verticals, beverages and K9AY-type antennas and more recently, arrays of active verticals.

Eham's own W8JI has created this sophisticated 8-element array available from DX Engineering for a few thousand dollars.
http://tinyurl.com/7hsllnt

Phased arrays of active RECEIVE-ONLY antennas offer QRM and noise reduction over a wide range of frequencies. Moreover, they are instantly electronically rotatable. Such arrays are perhaps the ultimate receiving antenna for real-estate restricted listeners.

Anyone here using sophisticated directional SWL antennas?

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EI4GMB
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Posts: 166




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« Reply #1 on: March 18, 2012, 10:23:16 AM »

Hi Jeff,

I would consider my RF Systems DX-1 Pro MK II Active receive antenna to be a sophisticated antenna. It is extremely well designed and is used in many embassies and military installations throughout the world. However, it is more omni-directional than directional and utilizes both vertical and horizontal polarization, so it performs well on multi-path signals. It has a high 2nd and 3rd order intercept point and is very quiet. It is also very sensitive. But it does not come cheap and at $700 is comparable in price to a beam!
I would be interested to hear your views.....

Best Regards

Fred EI4GMB
« Last Edit: March 20, 2012, 07:16:03 AM 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)
K0OD
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Posts: 2521




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« Reply #2 on: March 18, 2012, 11:38:05 AM »

Was glancing at the specs yesterday for that $700 Dutch listening antenna. It is said to have "10 db gain." Wow, right? That's about as good as a 5 element Yagi!   

But gain over what wasn't stated. I presume the preamp adds 10dB. So it has 10dB over itself with the preamp turned off... 10 dB over a small hunk of wire, in effect. Not so impressive.
http://www.rf-systems.nl/Shortform-catalog-complete.pdf

As for the military pedigree, it's pretty vague. In one place it says their antenna was "derived" from a military design. They never name the military or show a photo of it at war. Old trick is to donate something to a base ham club and then claim its "used by the military."

Full bore contest stations occupy many acres and have many antennas. Cost is a minor factor. You may be surprised that many of those antennas aren't monsters. They use small, low yagis, dipoles and verticals for spotting purposes and for back up and probably as references for testing the big antennas. They use many receive only antennas. They are constantly looking for an edge.

I'm inclined to think that W0AIH, for example, knows about actives antennas. I've never heard of him or any contest station using any of these SWL "wonder-antennas" (except perhaps in an array of them):
http://www.qth.com/w0aih/images/sunset.jpg
 
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EI4GMB
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« Reply #3 on: March 18, 2012, 12:20:39 PM »

Thanks for your comments Jeff. They are most interesting. You are correct about the 10dB gain. This is with the accompanying pre-amp active control unit. It can increase the QRN on a signal which is why I seldom use it. However, I think the IP2 and IP3 figures for the antenna are impressive and speak for themselves. The antenna is probably not as good as those used by full bore DX or Contest stations but because of its small size it is convenient for those BCB/SW listeners with small lots.
The mention of military and government users of the antenna could indeed be marketing hype. Maybe specific references to those users would add to the antennas credentials.
However, at the end of the day the proof is in the pudding and I must say the antenna performs well and is comparable to some of the longer beverage antennas I've heard used.
The antenna is particularly good on LW/MW and LF AM.
Indeed, I have heard a lot of great DX with it including some low powered SW stations from the australian outback.
Anyway Jeff, thanks again for your comments. I found them and the links you provided most informative.

73,

Fred EI4GMB
« Last Edit: March 20, 2012, 07:20:07 AM 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)
K0OD
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Posts: 2521




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« Reply #4 on: March 19, 2012, 06:20:25 AM »

Fred I couldn't recall reading about active antennas in ham literature. For one thing, they're strictly for receiving.  Also for traditional ham frequencies, the "random hunk of wire tossed out a window" is probably as good especially with a wide range tuner. Active antennas do seem to be useful below the AM broadcast band where hams worldwide are getting expanded authorization. 

HF Active antenna:
http://www.sentex.ca/~mec1995/circ/Activ8/activ8.html

LF Active antenna:
http://www.arrl.org/files/file/Technology/tis/info/pdf/0109031.pdf

I still don't understand how a tiny active antenna would work better than a hunk of wire or be worth $700. Here's a kit available for about $70. You'll notice that no extraordinary claims are made for its performance. It's sold mostly for those who can't put up any outside antenna.
http://www.northcountryradio.com/Kitpages/actant.htm

More interesting for hams would be arrays of active antennas now that commercial controller boxes are available. I used a 4-element system with transmitting and receiving capability on the 40 meter band years ago so I know that such arrays hear great.

Also,the major problem with arrays of transmitting verticals is the need for a good radial system under each vertical. With active receiving antennas a simple ground rod under each is all that's required.
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EI4GMB
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Posts: 166




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« Reply #5 on: March 20, 2012, 08:05:47 AM »

I still don't understand how a tiny active antenna would work better than a hunk of wire or be worth $700. Here's a kit available for about $70. You'll notice that no extraordinary claims are made for its performance. It's sold mostly for those who can't put up any outside antenna.
http://www.northcountryradio.com/Kitpages/actant.htm

That's a good point Jeff!
I have often thought the same thing about my G5RV Jr. when I hear the claims many antenna manufacturers make about their HF antennas!
Personally, I think this active antenna is overpriced which is probably why RF Systems have stopped producing them.
It was much cheaper when I bought it a number of years ago. I certainly wouldn't shell out $700 for it today, that's for sure!
I guess the advantage I have found using it, is that it allows me to pick up weak AM DX without intermodulation distortion or spurious signals.
Indeed when coupled to my Drake R8, it performs extremely well. I am unable to put up long beverage antennas yet I still get the same great receive performance, especially on LW/MW and LF AM.
Having said that, if I had to do it all over again I would probably buy something a bit cheaper! Grin

Best Regards

Fred EI4GMB
« Last Edit: March 20, 2012, 12:54:41 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)
K0OD
Member

Posts: 2521




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« Reply #6 on: March 20, 2012, 09:49:22 AM »

I can think of very few situations where an active antenna would make sense over a $2 hunk of random wire.  Perhaps it might be advantageous to mount an active antenna far outside a very "radio active" building like a factory or hospital, or a structure with lots of other transmitters like an embassy.  An active antenna on a high tower ABOVE such a building might help a lot in reducing crud from below. 

But the vast majority of noise is atmospheric QRN. A single active antenna won't reduce that. You just don't find ham DXers using active antennas much.

Note that DX Engineering sells a $289 hefty active antenna (designed by W8JI). It's only rated down to 100 KHz. It's designed to be used very close to transmitters as in a ham station. I'd trust that model over most of the SWL-targeted wonder-antennas.
http://www.dxengineering.com/ShowRelatedProd.asp?PartNo=DXE-ARAV3-1P

More on active antennas:
http://wiki.radioreference.com/index.php/Active_Antennas_KB6RWX

Which says in part: "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."

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EI4GMB
Member

Posts: 166




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« Reply #7 on: March 20, 2012, 01:12:34 PM »

I can think of one Jeff. When a full size (resonant) antenna or long wire would be too impractical to set up.
You have also mentioned some good instances yourself and provided some excellent links.
I think we may also be talking at cross purposes as you seem to be referring to HAM HF whilst I am referring to LF/MF BCB!
Anyway Jeff, I don't want to hog the topic. It has been an interesting discussion and I'd like to thank you again for your honest replies.
I will now leave it to others to give their 2 cents worth! Take care.

73,

Fred EI4GMB
« Last Edit: March 20, 2012, 02:38:44 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)
K0OD
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Posts: 2521




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« Reply #8 on: March 20, 2012, 04:13:39 PM »

Resonance is unimportant on receive. Regardless of HF Ham, BCB or LW. 

Otherwise we'd need 500' verticals on cars to hear AM BCB broadcasts. Sure a tiny car antenna has a ludicrously high SWR at say 540 KHz with resulting high coax loss per foot, but the coax is only a few feet long. And that small loss applies equally to the signal you want to hear and the noise you don't want to hear.  I think its fair to say that with a decent receiver you can totally offset those losses.... by turning up the volume control a bit. 

As for using an active antenna on extremely low frequencies, that makes sense in some settings.

--
Anyone else have comments?
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EI4GMB
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Posts: 166




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« Reply #9 on: March 21, 2012, 02:10:57 PM »

Resonance is unimportant on receive. Regardless of HF Ham, BCB or LW.  

As there are no takers I will respond.
A receiver antenna is less critical, since inefficiency can be compensated by signal amplification in the receiver, but there is improvement in reception when receiving antennas are tuned to resonance.
Also, amplification that occurs at the front end of a radio is not as good as amplification that occurs at the antenna end or signal source. Adding external gain can actually make the receiver more sensitive, but at the cost of dynamic range which is perfectly acceptable under certain conditions.
A good antenna factor is also needed to prevent overloading.
An active antenna may not be resonant on LF/MF BCB but high performance antennas like the RF Systems DX-1 Pro Mk II work by having a high quality, low noise, amplification circuit in place. They also have high IP2 and IP3 which means less intermodulation distortion and spurious signals.
This together with the use of a good radio receiver can make all the difference when trying to hear weak AM stations or DX on LF/MF!
 
« Last Edit: March 21, 2012, 05:08:23 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)
KB1GMX
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Posts: 711




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« Reply #10 on: April 03, 2012, 04:18:50 PM »

>Resonance is unimportant on receive. Regardless of HF Ham, BCB or LW. 

Not completely but depends on the system goals.  Resonance often yields greater signals
with less noise due to band width reduction.

>Otherwise we'd need 500' verticals on cars to hear AM BCB broadcasts. Sure a tiny car antenna has a ludicrously high SWR at say 540 KHz with resulting high coax loss per foot, but the coax is only a few feet long. And that small loss applies equally to the signal you want to hear and the noise you don't want to hear.  I think its fair to say that with a decent receiver you can totally offset those losses.... by turning up the volume control a bit. 

First car antennas are connected with high impedance low capacitance cables directly to the first tuned circuit.  It's a voltage probe rather than short(end) whip.

>As for using an active antenna on extremely low frequencies, that makes sense in some settings.

Most common example is the WWVB at 60khz, and before it was shutdown the LORAN-C
active antennas at 100Khz.  Any thing bigger on a airplane has high drag costs.

Years (many) ago I used to play in the 1750M band (160-190khz part 15).  Receive
antennas were not restricted so a loop of wire nearly 1000ft long was used(with tuning and matching) and the transmit antenna was limited by rule to 50FT total cable and antenna
so the tx was at the bottom and part of the big load coil(many turns of #8) pumping
a mighty 1W input.  AM modulated signals were good for about 50 miles, SSB more.
We didn't have computers back then so QRSS was not done.

--
>Anyone else have comments?

Ask you get.


Allison

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AUSSIE
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Posts: 38




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« Reply #11 on: May 05, 2012, 05:48:20 PM »

Hi Rfsytems & Wellbrook active antennas do an excellent job nulling out interference
expensive but u get what u pay for in this hobby..

Regards Lino..
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K0JEG
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Posts: 633




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« Reply #12 on: May 06, 2012, 08:07:39 AM »

Resonance is unimportant on receive. Regardless of HF Ham, BCB or LW.  

As there are no takers I will respond.
A receiver antenna is less critical, since inefficiency can be compensated by signal amplification in the receiver, but there is improvement in reception when receiving antennas are tuned to resonance.
Also, amplification that occurs at the front end of a radio is not as good as amplification that occurs at the antenna end or signal source.

Proof of this can be found with magnetic loop antennas. When manually tuning them you get a rough adjustment by listening to the loudness of noise or signal and peaking the S-meter.
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K0OD
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Posts: 2521




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« Reply #13 on: May 06, 2012, 09:07:46 AM »

Yes, signal and noise both peak at resonance. An S-meter will rise but the DX station is no easier to copy amid the louder noise.  Low band DXers agonize over many things, but not about tuning their receiving antennas for precise resonance.

For best reception, would you suggest that avid 160m ops put up a bunch of antennas, each tuned to a slightly different frequency on the band? That would be quite a waste.
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AC5UP
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Posts: 3822




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« Reply #14 on: May 06, 2012, 10:05:58 AM »

Do a little web search and you'll find plans for resonant loops using varactor diodes as the tuning caps. Nice concept as a DC voltage can be superimposed on the feedline and there are no moving parts anywhere north of the control box. Might see a slight reduction in signal due to the DC block on the RX end of the line, but it has been my experience that even a modest loop is capable of bodacious signal levels.

Years ago I experimented with square loops four feet on a side and wound with seven turns of 20 ga solid enameled copper. The target band was AM broadcast and they worked well enough on local stations to overload almost every receiver I tried with them. At night the entire band was S9+ from end to end and LOUD. At first glance this might seem like a good thing, but in reality the quality of reception was much like any consumer radio with a decent ferrite rod antenna - except - the AVC had more margin on fades.

Otherwise it was the same ol' same ol' of three or more simultaneous stations on almost every channel......... A GE SuperRadio II will demonstrate that quality straight out of the box with no need to peak a second knob.
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