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

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Antenn spacing  (Read 344 times)
KC1ILJ
Member

Posts: 7




Ignore
« on: December 05, 2017, 02:15:00 PM »

Hello,

How far should I space this antennas?
2 Meter Slim Jim
UHF 440-470 MHz Slim Jim
222MHz Amateur Radio Slim

I’m gonna install them in the attic and connect them to a alpha delta antenna switcher with some lmr 400uf
Got them from KB9VBR J-Pole Antennas. He said in regards to the spacing 1/4 wavelength of the lowest freq  at the min but best to be as far as I can.

Other are saying  1/2 of the wavelength so 1m min

Also for those antennas above are there any restriction/ considerations in regards to the length of the cable? I’ve seen some complex calculations where 1/4 wavelength was multiplied by something then divided but smth else and you were supposed to get the length only in multiples of that.

Thanks!


Logged
WB6BYU
Member

Posts: 17176




Ignore
« Reply #1 on: December 05, 2017, 03:07:52 PM »

Quote from: KC1ILJ

How far should I space this antennas?
2 Meter Slim Jim
UHF 440-470 MHz Slim Jim
222MHz Amateur Radio Slim




How much room do you have?  How many supports do you have to mount them on?

It really depends on what you are trying to avoid by spacing them apart, so a single numerical
answer is likely to be misleading.

For example, if you were using them on two different radios that you wanted to use at the
same time, then you would need to space them apart far enough that transmitting through one
doesn't damage the receiver connected to the other.  A different (and more difficult) objective
would be to space them so one transmitter didn't desense the other receiver - that may require
much wider spacing.

In your case, if you are just trying to prevent the other antennas from disturbing the SWR of the
other antennas, then 1m or so is probably adequate.  You may still have some pattern distortion,
especially as the taller antenna tends to reflect the signal from the shorter ones.  You might need
2 to 3 times the distance in that case (or more depending on what level of pattern distortion is
unacceptable.)

For general FM work through local repeaters, half a wavelength on 2m is probably reasonable.
That's not to say that you couldn't get by with 24" perhaps - life is all about trade-offs.  If you
are trying to mount them all on the same mast, then things may need to get more creative.




Quote

Also for those antennas above are there any restriction/ considerations in regards to the length of the cable?



The best cable length is the shortest distance between the antenna and the transmitter.
Certainly don't go any shorter than that!


I've seen a number of other cable length formulas over the years.  They are usually based on
questionable or false assumptions - if, indeed, they are based on anything other than mythology.

Note that J-pole type antennas (including the Slim-JIM) are often affected by common mode
current (that is, the coax shield becomes part of the antenna and radiates.)  Some cable lengths
may be more susceptible to this than others, but also it depends on what else in your system is
connected to the coax.  If that's a problem, you need to fix it at the source rather than trying to
find a cable length that disguises the problem.
Logged
KC1ILJ
Member

Posts: 7




Ignore
« Reply #2 on: December 05, 2017, 03:36:20 PM »

I’ll be using only one radio.

Each Antenna will be mounted on its own pvc pipe and the pipes mounted on the attics floor (unfinished floor so I can clamp them with U clamps to the floor rafters )
Attic is pretty spacious length wise and about 6 ft at the peak.
Logged
N8AUC
Member

Posts: 318




Ignore
« Reply #3 on: December 06, 2017, 08:54:59 AM »

I’ll be using only one radio.

Each Antenna will be mounted on its own pvc pipe and the pipes mounted on the attics floor (unfinished floor so I can clamp them with U clamps to the floor rafters )
Attic is pretty spacious length wise and about 6 ft at the peak.

One radio? At the same time?
If you're only using 1 radio in total, 1 wavelength should be fine.
If you want multiple radios at one time, and not have them interact, you'll need more spacing.
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!