DIY antenna - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th May 2007, 08:29 PM   #1
Fry-kun is offline Fry-kun  United States
diyAudio Member
 
Join Date: May 2007
Default DIY antenna

Hi there,
I'm trying to make an FM antenna for my receiver (Onkyo TX-SR604) using these instructions.
Hopefully it'll be enough for my needs... though it looks like I need a 300->75 ohm converter (and apparently those decrease the signal quality)

Anyway, my question is more general: it seems from the antenna design that the wire is looped - which raises the question "where does the 300ohm come from?". If the wire is in a loop, the resistance is barely anything... or am I missing something?


TIA for any info
  Reply With Quote
Old 9th May 2007, 10:07 AM   #2
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
Yea,the DC resistance will be quite low,but the impedance at ~100mhz will be somewhat higher (somewhere around 300 ohms.)
  Reply With Quote
Old 15th May 2007, 10:58 PM   #3
diyAudio Member
 
Join Date: Jun 2004
Location: Wylie, Texas
A little more info....

The antenna impedance is a combination of the resistive loss (similar to DC resistance, but not exactly) and the radiation resistance, which is the load caused by the actual radiated power.

The basic antenna is a halfwave dipole. Impedance is 75 Ohms. To find the length, divide 5904 by the freq. in MHz. About 60 inches for 98 MHz. Cut the wire in half and feed coax to the middle (center to one side, shield to the other). Good antenna, but it does have a null along the axis of the wire.

A Loop or Folded Dipole (basically a squashed loop) will split the current between two halfwave sections, thus causing the feedpoint impedance to increase by 4 times; 300 Ohms. A wonderful thing when using 300 Ohm twinlead, but not so great for coax. The losses in a 300 to 75 Ohm transformer will probably eat up what little you gain in the loop. And the loop will still have the same polarization and null issue.

And, Yes, I do work in the field. I love antennas....

Have Fun,
Kent
  Reply With Quote
Old 24th May 2007, 08:52 PM   #4
diyAudio Member
 
Join Date: Sep 2001
Location: Northern California
Default Antenna

I Vote J-Pole... or several 1/2 waves in phase.

Just about as easy to build


Cyclotronguy
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
FM antenna coldcathode Everything Else 6 16th June 2009 05:20 PM
car antenna coki Car Audio 3 11th August 2007 11:35 AM
FM Antenna jaya000 Car Audio 5 27th January 2007 04:40 AM
Antenna ? fortuosity Everything Else 1 17th October 2005 10:14 AM
What do you think about this antenna ? darkm4n Parts 0 6th March 2004 09:47 PM


New To Site? Need Help?

All times are GMT. The time now is 11:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2