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gzimmerPosted - 28 February 2013 1:4  Show Profile  Email Poster  Edit Message
Crystal sets are fun, but for many city dwellers, erecting a long wire antenna is not possible.

For a long time I've been meaning to try an Active Antenna on a Crystal Set.

There are a number of different designs on the net, but recently I made up a PA0RDT Active Antenna.

see
http://dl1dbc.net/SAQ/miniwhip.html
and especially the references at bottom of that page.

For a picture see
members.wideband.net.au/gzimmer/Misc/VK2DDI_Mini-whip_02.jpg

Or do a search on PA0RDT Active Antenna.

There is even a kit on ebay
http://www.ebay.com.au/itm/Activate-antenna-10kHz-to-30MHz-kit-Vlf-mf-hf/111019734031?ssPageName=WDVW&rd=1&ih=001&category=4672&cmd=ViewItem

Now to the results:

The Active Antenna appears as a low impedance resistive source, so it is easy to match into your crystal set. I used a simple 1 turn link on my toroid set. For a conventional set, just put the coil and tuning condenser in series across the output of the Active Ant.

When connected to a crystal set, my Active Ant performs about the same as a 20ft wire antenna. Although I could hear all the local stations, I though it a bit deaf at first, so next I built a simple 12db pre-amp using an ERA-6 chip (see data sheet notes). This gave signals a big boost. This combination was about equal to my 60ft antenna.

Then I built a 22db MAR-6 pre-amp. This probably gave a bit too much gain as it caused some overload. Was louder than my 300 Ft long wire.

Note these MMIC chips I used are a bit of an overkill as they are really intended for VHF and microwave use. They are however cheap and readily available on ebay. You could certainly build something equivalent with conventional components.

The big advantage of an Active Antenna is that with light coupling it can give you great Selectivity with one-knob tuning.

So the bottom line is that the PA0RDT Active Antenna is a very handy thing to have for any kind of BC and SW radio listening, especially if you can't put up a long wire.


later ......... Zim


Edited by - gzimmer on 2/28/2013 8:50:57 AM

John Bruce McCreathPosted - 28 February 2013 7:19  Show Profile  Email Poster  Edit Message  
Take a look at the site below for another option on active antennas. I have both and they work amazingly well for their size.

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

73, J.B., VE3EAR

Into radios since 1959.

golfguruPosted - 28 February 2013 14:54  Show Profile  Email Poster  Edit Message  
Link Does not work for me.
Goes to here:
http://www.arrl.org/img/file_types/missing.png
(Displays ARRL logo).

..............

gzimmerPosted - 28 February 2013 21:8  Show Profile  Email Poster  Edit Message
It works if you delete the full-stop at the end

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

........Zim

Electronic TechnicianPosted - 1 March 2013 21:49  Show Profile  Email Poster  Edit Message
In reference number 2 for the PA0RDT Active Antenna he places the active antenna on a pole. In that case the feedline feeding the active antenna is part of the vertical antenna. It is a common mistake. If you don't duplicate this part, you don't get the gain he has. I use a J310 whip for 160 meter reception. I work a 4 foot ground mounted whip attached to my mail box in the front yard against a ground rod. I use transformer coupling to reduce pickup from the feedline. This is about the longest antenna I can use without BCB overload. I use 3 J310's in parallel to increase output power. The output impedance is then low enough to drive a 50 ohm radio. MMIC's are great. Just make sure you use a good one that has 1db compression around +17dbm or more. The cheaper ones overload easily. MAV11 is a good one. Be sure and try different locations for your active whip. Some locations are much quieter than others.
gzimmerPosted - 1 March 2013 22:18  Show Profile  Email Poster  Edit Message
Deleted

Edited by - gzimmer on 3/1/2013 10:24:11 PM

_J_Posted - 5 March 2013 22:19  Show Profile  Email Poster  Edit Message
I collaborated offline with Ben on a design for an active BCB antenna for kid groups. I finally agreed with him that the needed gain bandwidth is the main challenge. We designed and did thought experiments but didn't build / test anything.

John Davidson

Edited by - _J_ on 3/5/2013 10:21:21 PM

gzimmerPosted - 5 March 2013 23:33  Show Profile  Email Poster  Edit Message
> I finally agreed with him that the needed gain bandwidth is the main challenge.

John, what does this mean in practice please?

The frequency response of these Active Ants is quite flat, and it's easy to add more gain if necessary...

.......... Zim.

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