Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: SWL antenna for attic  (Read 5659 times)
KK4DPN
Member

Posts: 22




Ignore
« on: July 26, 2015, 11:34:14 AM »

Hello everyone,

I'm wanting to put some sort of coax-fed receive-only antenna in my attic. I've used longwire antennas in the past connected directly to a receiver and had decent reception, but I know I can improve upon that.

The receiver I'm using now is an RTL-SDR dongle and a Ham-it-up upconverter. I've had a few people recommend using a 9:1 balun, but I have a few questions.

Do I have to ground the balun to something in order for it to work correctly? There's not anything to ground to in my attic that I'm aware of.

Logged
AA4PB
Member

Posts: 13774




Ignore
« Reply #1 on: July 26, 2015, 12:45:49 PM »

I'd center feed the wire antenna like a dipole rather than trying to end feed it with coax. You could use a balun at the feed point.

Logged
KB4QAA
Member

Posts: 3062




Ignore
« Reply #2 on: July 26, 2015, 08:22:31 PM »

I would put up a loop or dipole.
Logged
KK4DPN
Member

Posts: 22




Ignore
« Reply #3 on: July 26, 2015, 11:47:40 PM »

I'm thinking a dipole will be best then.

What frequency should I build it for? 15mhz? Something right in the middle of what I want to listen to?

Also, if I run short on space is it alright to have the antenna make turns in order to be fully stretched out? Like if I hit a wall can I just make a right angle turn and keep going or what?

Thanks for the advice.
Logged
AA4PB
Member

Posts: 13774




Ignore
« Reply #4 on: July 27, 2015, 09:17:10 AM »

Since it's just for receiving, the exact frequency is not very important. 15Mhz is probably good but you can adjust the length to whatever fits. Making a turn won't be much of a problem but may not be necessary. 15MHz is 31 feet end to end so if you can only get 25-30 feet in the space, I'd just go with the length that fits.
Logged
KK4DPN
Member

Posts: 22




Ignore
« Reply #5 on: July 28, 2015, 01:11:45 PM »

Will do. Thanks for the help!
Logged
K5TED
Member

Posts: 1188




Ignore
« Reply #6 on: July 28, 2015, 05:38:10 PM »

9:1 is generally used with long random wires connected by coax. If you look at a 9:1 balun diagram, you will notice the ground side of both windings is connected to the coax shield.   Grounding at the feedpoint may change the noise characteristics on some bands, and have absolutely no effect on others, and little effect on receive signal strength.

For outside installations, it's hardly ever a bad idea to ground the balun so as to bleed off potentially destructive static electricity buildup on the long wire.

In you attic, maybe a multi-turn wire loop, say, 5 times around the attic, suspending the wir with cheap d-rings and using a 1:1 balun to feed the coax might give outstanding results.

Logged
PITSWL
Member

Posts: 111




Ignore
« Reply #7 on: August 25, 2015, 07:07:32 AM »

On a related note, and regardless of whether or not this is an indoor or outdoor installation, what is the real effect of proximity to power lines? Lines running up and down your walls to outlets obviously can't be moved, but if you have lines running to light fixtures and such, how far away from your antenna should they be to avoid any RFI that might be generated?

Or, put another way, just how much RFI does your basic power line running to, for instance, a light fixture put out that will be picked up as noise by your antenna?
« Last Edit: August 25, 2015, 07:09:39 AM by PITSWL » Logged

That man is playing Galaga!
K3WVU
Member

Posts: 497




Ignore
« Reply #8 on: September 27, 2015, 11:41:14 AM »

I remember an article in the old "Electronics Illustrated" magazine about a multiband SWL attic antenna made from 300 ohm twinlead.  Don't recall which issue, but all of the Electronics Illustrated back issues can be read here:  http://www.americanradiohistory.com/Electronics%20_Illustrated_Master_Page.htm

Logged
AA4PB
Member

Posts: 13774




Ignore
« Reply #9 on: September 27, 2015, 11:52:49 AM »

Or, put another way, just how much RFI does your basic power line running to, for instance, a light fixture put out that will be picked up as noise by your antenna?

There are so many variables that no one can give you a magic number. I've seen an arcing connection on a power pole that generated RFI which traveled nearly a mile down the line and into a building. On the other hand, I had an attic antenna that was only about 2 feet from electric lines and I received no RFI.
Logged
KB4QAA
Member

Posts: 3062




Ignore
« Reply #10 on: September 27, 2015, 04:05:29 PM »

For receiving, the antenna dimensions and type are really unimportant.   

For general purposes, a random length wire 25ft - 50ft wire was recommended by multiple manufacturers for decades.

A doublet/dipole or loop which are all balanced, will all perform admirably.  A 1:1 current balun at the feed point (for dipole or loop fed with coax) can reduce common mode RFI pickup, but is optional.    Make the antenna whatever size fits your situation or if you have a specific band of interest. 

Dipole/Doublet =  468/Freq in Mhz = total length in feet
Full wave loop = 1005/Freq in Mhz = total length in feet.

NOTE:  For general coverage SW receivers (i.e. non-ham/military) there is often a hazard of having too strong a signal!!!!   

Many of these consumer level radios do not have wide dynamic ranges and can be overloaded and give poor signal quality performance when given too high a signal strength.   The RTL Dongles are especially noted for poor dynamic ranges!!!!!  I recommend you read up on this topic and particularly for your device.

In these cases, "Less is More".   Use a shorter/smaller, lower gain antenna if overloading occurs.  Alternatively, a stepped attenuator could be used to have the advantage of better antenna gain, while also being able to adjust signal strength to a usable level.
Logged
PITSWL
Member

Posts: 111




Ignore
« Reply #11 on: October 28, 2015, 07:45:23 AM »

Thanks for the replies, KB4QAA and AA4PB!
Logged

That man is playing Galaga!
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!