Alford Slot – Horizontal Omnidirectional Antenna

September 13th, 2007


Alford Slot – Horizontal Omnidirectional Antenna working on HAM band – somewhere around 2.3ghz or 13cm.

Whit several words what is Alford Slot : this is circular pipe whit ONE LONG slot on it, feed by coax line coming from inside the tube.

Here is some photos :

Alford SlotAlford Slot

note pics are for lower frequency – typically for 1200Mhz, second shows that for low band this antenna can be made even from wire

The tutorial how to make this antenna is on this site : http://www.g8ajn.tv/23cms.htm and http://www.k5rmg.org/Alford-slot.html

Interesting about this antenna :

Omnidirectional paten

Horizontal polarization

Physically its circular – for me its important omni antenna to be circular – do not ask 🙂

Interesting feeding point of slot.

The design of this antenna is based on several tables or ready pre-calculated values.

Freq.
(MHz)
Slot Length
2.15 l
(inches)
Active
Circum.
0.40l
(inches)
Slot Width
0.016l
(inches)
Final
ant.
dia.
(inches)
902.1 28.25 5.20 0.190 ~1.72
1296 20.125 3.910 0.170 ~1.3
2304 10.688 2.120 0.090 ~0.7

active-circumference.gif

Here is some pictures from www.g8ajn.tv

cameradownload-071.jpgcameradownload-072.jpg

cutting the slot and ready feed points

Here is the interesting part – building of the feeding point I was really surprised :

cameradownload-073.jpg cameradownload-074.jpgcameradownload-075.jpgcameradownload-078.jpgcableties.jpg

Only this is for separate post ! 🙂 This showing how to make impedance matching directly whit coax ! So on this on the last picture coax line is impedance matched.

cameradownload-082.jpg cameradownload-080.jpgcameradownload-079.jpg

Next is connecting coax to the slot, second picture is fine tunning, and the third is preparing for mounting. Tunning is made by extension of the slot.

Go on the sites shown up and read the instructions for building this ones You may find thing that I missed.