Re: 5/8th Wave Antennas Why does the 5/8th have gain, and how much gain does it have?


Greg - KO6TH
 

Good info, Jef.  I am running a 5/8 wave J-pole for my packet station, and it works pretty well.

A follow-up to the satellite question..  One generally wants some sort of gain, such as a small beam.  Satellites are relatively weak, since they're hundreds to thousands of miles a way (line of sight), and generally don't have much by way of transmit power.  The exception is the International Space Station, which is in a fairly low orbit, and has a transmitter with some oomph behind it (up to 25 w, but they probably run at a bit lower power).  SO-50 is one of the weaker ones at 250mw, and many of the "cubesats" aren't running much more.

All that said, one of the better antennas can be a simple 2m 1/4 wave ground plane antenna, operated in dual-band mode.  On it's 3rd harmonic fro 70cm, it turns out to have some gain upwards from the horizontal, which helps with satellite work.  It's still not wonderful, but a LOT better than an HT's antenna, and can sometimes be better than a high gain omni antenna, where the pattern is aimed at the horizon. 

http://aprs.org/aprs-satellite-igate-antennas.html  (Old satellite and product info, but physics still works.)

Greg  KO6TH


Jef - N5JEF wrote:

Tonight someone on the Elmer net was asking about ⅝λ antennas with regard to a ground plane.  I tried to explain that a ⅝λ antenna is already a complete antenna, so it doesn't require a ground to make it complete the way a ¼λ wave antenna does.  Further, a ⅝λ antenna gets its gain (lower-angle radiation) from ground reflections further out from its base.

Here's a page that I think explains ⅝λ antennas very well and clears up some myths and folklore:  https://www.w8ji.com/VHF%20mobile%20vertical.htm

As a related matter, it seems that few people realize that a ¼λ whip with a decent ground plane has 3dB gain over a full-length vertical dipole twice its length.

- Jef N5JEF

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