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

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: short 40 meter antenna  (Read 5131 times)
VK4TJF
Member

Posts: 101




Ignore
« on: December 13, 2014, 09:51:16 PM »

hello I would like to make a short dipole for the 40 meter band
i would like the antenna to be only 10 meters in length
and made of aluminum
i would like to use linear loading and loading coils
is there any one there that can draw something up for me?
Logged
WX7G
Member

Posts: 6456




Ignore
« Reply #1 on: December 14, 2014, 12:54:31 AM »

Loading coils are less lossy than linear loading so let's look at that.

With loading coils at the feedpoint they need to be around 12 uH. Placed half way along each dipole half they need to be around 21 uH. Assigning an inductor Q of 200 to both configurations there is virtually no difference in gain.
Logged
VK4TJF
Member

Posts: 101




Ignore
« Reply #2 on: December 14, 2014, 02:00:05 AM »

so your saying that i would just need two loading coils 21uH each
one for each side then
Logged
VU2NAN
Member

Posts: 268


WWW

Ignore
« Reply #3 on: December 14, 2014, 06:19:55 AM »

Hi OM Jim,

Here's one called the 'Shorty Forty'!

http://www.qsl.net/on7eq/projects/shortyforty.htm

73,

Nandu.
Logged

WX7G
Member

Posts: 6456




Ignore
« Reply #4 on: December 14, 2014, 07:20:08 AM »

Yes 21 uH on each side mounted 2.5 meters from the feedpoint. That is for 1 mm wire. The element diameter greatly affects the loading beyond the inductors and must be known so the correct inductance is found. For example, with 25 mm tubing the inductors are 12.5 uH.

For the 25 mm version, given inductor Q of 20,0 the input resistance at resonance is 38 ohms, the efficiency is 84%, and the 2:1 VSWR bandwidth is 140 kHz. It can be fed direct with a minimum VSWR of 1.3:1. The easiest way to adjust the antenna is to change the element length from the inductors outward.

I like to place the loading inductors at the ends as I find that easier mechanically.  The spoke lengths are used to adjust the antenna to resonance. For that top hats are used. With four 75 mm x 3 mm spokes on each end the end inductors are 14 uH. The input resistance at resonance is 43 ohms, the efficiency is 89%, and the 2:1 VSWR bandwidth is 160 kHz. More top hat wires or longer wires give wider bandwidth (by reducing the antenna reactance).
Logged
KQ0C
Member

Posts: 78




Ignore
« Reply #5 on: December 31, 2014, 09:09:07 AM »

You will get large efficiency gains from the antenna if you let wires hang down from the ends of the aluminum. Lets say your antenna is mounted at 10 meters... add wires that hang down 5 meters and you need almost no coil loading. You will have made an end loaded dipole similar to the EVO Tennas coming out of the UK. It doesn't matter that the wire blows around.
Logged
VK4FFAB
Member

Posts: 44




Ignore
« Reply #6 on: January 02, 2015, 05:55:58 PM »

hello I would like to make a short dipole for the 40 meter band
i would like the antenna to be only 10 meters in length
and made of aluminum
i would like to use linear loading and loading coils
is there any one there that can draw something up for me?

Sounds like a Buddypole James,

Cheers,

Rob.
Logged
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!