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 Post subject: Active Antennas
PostPosted: Nov Fri 17, 2017 4:26 pm 
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Joined: Jun Fri 09, 2017 1:54 pm
Posts: 2
Am I in the right group for active antennas?

If so, there is Sony AN-1 on Ebay of the sort I used with great success in the 1980's, but in 2017, there is frequently bad reception, RFI from neighbors et al.

Any known antenna close to results An-1 yielded? Tnx from N3FSH/


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 Post subject: Re: Active Antennas
PostPosted: Nov Fri 17, 2017 5:57 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20691
Location: Annapolis, MD
In the (good old) days of shade-tree hot rodding, there was a mantra: "there is no substitute for cubic inches".

The equivalent parameter for antennas is aperture. As a general rule you cannot trade gain for aperture. For an amplifier to increase sensitivity, it has to have a lower noise figure than whatever it is feeding...and it has to have efficient coupling and shielding, etc.

On average an amplified antenna is a waste of money. I'm sure there are exceptions.

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 Post subject: Re: Active Antennas
PostPosted: Nov Fri 17, 2017 6:22 pm 
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Joined: May Fri 04, 2012 10:39 pm
Posts: 1240
Location: Hidden Valley, AZ
This one is worth every penny.. :D

http://www.loop-antennas.com/ALA1530LN-2

DG

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 Post subject: Re: Active Antennas
PostPosted: Nov Fri 17, 2017 6:27 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
Good example of an exception....by having an amplifier at the antenna, you reduce the effects of noise pickup on the leadin.

I would think a really good tuned circuit would do almost as well.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Active Antennas
PostPosted: Nov Fri 17, 2017 6:39 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4817
An amplified antenna can work well in today's RFI filled world if you locate the probe where it is away from all of the RF garbage created by household items and present on the power line. Some of the really good VLF setups use an active antenna with antenna mounted preamp/impedance converter at the antenna assembly and this is located far away from the house and any wiring.

In the current RF (or RFI) environment the key is to get the antenna away from noise sources so if you have an active antenna look at some locations that appear far removed from noise sources and try several of those to see which location works best.

I am in a very RF quiet rural location but a few years ago I had an intermittent noise source popping up and it was imposed on the power lines so I was using a small loop antenna for 80 meter receive with a null placed on the power line source. It was a homemade loop, 2 feet per side, with a battery powered impedance converter (single FET) at the base so that the shunt capacitance of the connecting coax line wouldn't tremendously attenuate the signal from the very low impedance loop source. I fed the loop and the output from the T/R switch connected regular antenna to a MFJ noise canceller (phase shift box) that I had purchased in an initial attempt to make the noise go away. I just used the MFJ as a preamp to bring the small loop output up to near what my regular transmitting full wave 80 meter loop provided and to choose between them on the rare occasion that a desired station was in the null of the loop.

The small loop output was a fraction of what my regular antenna provided but the signal to noise ratio made it the clear winner when the noise popped up. A well placed active antenna can provide similar results in a RF noisy environment.

Rodger WQ9E


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 Post subject: Re: Active Antennas
PostPosted: Nov Sat 18, 2017 2:36 am 
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Joined: Sep Thu 01, 2016 3:56 am
Posts: 120
Location: San Jose, Ca.
Joel, I have a Welbrook ALA1530LN and am very happy with it. I'm currently real estate challenged, as I live on a standard city block in San Jose, Ca. The 1st antenna I put up here was a 40' vertical with a decent grounding system and my reception was surprisingly good. I was used to horizontal wire dipoles at my last home, which was of course better than the vertical, but the s/n on the vertical was still quite good and the noise was not very bad.

When I put up my 1st Welbrook ALA1530 (not LN) loop, it provided a little better s/n (about 9 db) than the vertical and less noise. Then Welbrook came out with the ALA1530LN, and the specs sounded like it would be worth a try, so a year ago I got one and it is better. I had both of the loops up for a while so I could do some comparisons. The 'LN' is about 3db better in s/n than the ALA1530 below 13 mh and 1 db above. I kept the LN and am very happy with it. Low noise levels is one of the great benefits of these loops.

Regards, Larry


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 Post subject: Re: Active Antennas
PostPosted: Nov Sat 18, 2017 5:09 pm 
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Joined: Feb Sun 10, 2013 3:31 am
Posts: 43
The "Mini-Whip" active antenna at the University of Twente WebSDR works pretty well. It is based on the popular PA0RDT voltage probe design (it is not a magnetic loop, rather it is a small rectangle of PC board attached to the amplifier on the mast.) I often listen to 40 meters using the university's WebSDR site via the internet when the gray line is open from the Netherlands to the east and midwest US. Earlier in the afternoon/evening, a lot of stations to the east of Europe, Africa and Asia on 40 and 20, when it is open.

Photos of the antenna at:

http://websdr.ewi.utwente.nl:8901/qrt.html

Main site:

http://websdr.ewi.utwente.nl:8901/

Pieter, PA3FWM's site, showing the evolution of how he homebrewed the SDR between 2004 to present:

http://wwwhome.cs.utwente.nl/~ptdeboer/ham/sdr/

Ted, KX4OM


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