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: QRP 20 mtr vertical - no ground required  (Read 313 times)
N1GMV
Member

Posts: 68




Ignore
« on: September 23, 2003, 08:13:06 PM »

I am trying to build a qrp vertical antenna. There are a number of reasons why I could use this antenna instead of my dipole but in this case I will be operating from the beach.

I was looking at all the antennas out there on the market for qrp operation and simply cannot justify the cost so decided on building my own.

My current construction consists of a Camera Tripod that supports a 1/2" PVC pipe that separates into two sections. I am using about 8' of tubing. I decided to take 1/2 wavelength of wire and make a helical winding up and inside the the pvc using quick-disconnect plugs at the separation points.

Now, time to make it resonant… I am not even close, seems like the vertical whip is close to 10Mhz instead of 14! I am trying to use a toroid at the bottom and have the feed point be on one of the taps, the radiator on the end and the ground on the other end of the windings.

I cannot find any plans to go by to tell me when I should trim or lengthen the radiator and when to adjust taps on the toroid. Maybe I should simply move the feedpoint up the helical and put connect the sheild to the bottom. Is there any way to measure or is this simply trial and error?

Any suggestions would be helpful, I will try and post a photo here soon.

Thanks;
John, N1GMV


Logged
W6HB
Member

Posts: 38




Ignore
« Reply #1 on: September 23, 2003, 08:42:45 PM »

If, in fact,you are showing resonance at 10 MHz, you are too long. I am having a hard time drawing a mental picture from your description. I might suggest you check out www.cebik.com for many tried and true solutions and ideas.
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #2 on: September 23, 2003, 09:10:40 PM »

Hi John,

The exact length of wire all coiled up in the antenna has nothing to do with resonance. It all depends on how and where the wire is coiled what the resonant point will be. Resonance by wire length only works accurately when the wire is not folded or coiled into a small area.

The antenna also really is not a half-wave, if it is ten ft long it is a ten foot vertical even if you pack 1000 feet of wire inside!

Unless you have a ground system on that vertical and BASE feed the antenna, you are in for major feedline decoupling and efficiency problems. It is virtually impossible to center feed a vertical like dipole and not have RF on the feedline without major care to the antenna design.

While helical loading is not very efficient loading, you should still be able to make contacts with the loss in efficiency. You might consider some alternatives. I'd recommend reading the ARRL Antenna Handbook, and staying away from Internet or data that isn't peer reviewed. You might be able to do a good inductor, and a capacitance hat of some type. That would be far more efficient.

Did you consider a dipole or Inverted Vee dipole??? A dipole would be far more efficient and easier to feed.

73 Tom  
Logged
AA4PB
Member

Posts: 12832




Ignore
« Reply #3 on: September 23, 2003, 09:25:44 PM »

The first problem is that you cannot just take a 1/2-wavelength of wire and coil it up to form a loading coil. It won't be resonant on the intended frequency because of the inductance of the coiled wire. The loading coil works by creating the amount of inductive reactance that exactly cancels the capacitive reactance of the short antenna, making it resonant.

The second problem is that an end fed antenna, even if it is 1/2 wavelength must work against some type of grounding system. While a tapped coil can provide the necessary impedance match, you can't just leave the bottom of the coil and the coax shield floating above ground. It must be connected to some type of ground or radial system in order to work with any efficiency at all. One option is to come up with a means to center feed the 1/2-wave vertical so that the antenna is balanced (like a dipole turned on end).

A pair of 20M hamsticks formed to make a shortened dipole makes a good portable antenna but it make not break down into short enough pieces for easy packing.
Logged
RobertKoernerExAE7G
Member

Posts: 1435




Ignore
« Reply #4 on: September 23, 2003, 09:31:53 PM »

Beach?  As in salt water?

Even if it is fresh water, I think you are making your life too hard.

I would think one wants a LOW loss antenna for QRP.

What couple of bands do you want to get on? 20 and 30 or 40?

Bob
Logged
K0ZN
Member

Posts: 1547




Ignore
« Reply #5 on: September 23, 2003, 10:12:10 PM »

Hi,

If you are operating QRP you should make every possbile effort to avoid losses. It you are running a KW and lose 15% you will never ever notice it, but with QRP, "every drop counts" (big time!) Stick with a full size antenna if at all possible. Unless you can absolutely, positively get a GOOD saltwater ground, I would not mess with a base fed vertical; verticals require a really good ground radial system to be effective. A little work and creativity and you should be able to come up with some kind of easy to put up mast that would hold the center insulator for an inverted V. If size is a major issue, you can fold each side of the antenna back upon itself making it 1/2 normal length, but a full half wave of wire... this is called Linear Loading and is somewhat lower loss than lumped constant loading.

Bottomline: a really small antenna probably won't work very well on QRP. Dig into one of the better antenna books for some options.

73
Logged
N6AJR
Member

Posts: 9906




Ignore
« Reply #6 on: September 24, 2003, 12:10:13 AM »

two words   Fan DIpole..Smiley
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!