Message from Dreamxtreme at Friday, 17-Sep-10 04:21:32 GMT

Hi All

Wonder if you can help me

I'm going to be putting up 2 FM8's temporarily on a big pole in the back garden

However I'm having trouble finding any information on spacing and coax length

Basically my thoughts are I have the 2 beams on a horizontal pole to put both beams on side by side

Like this
http://www.ham-radio.com/k6sti/small9.jpg

I cut the pole down so the beams are at both ends.
And as these are direct feed I can add the coax to the 2 feed points on the left antenna and attach it to the 2 feed points on the right antenna which is where the main coax to the radio is also attached.

The beams are going to be Horizontally polarized. So a few question from this

1. Will that work ??
2. How far apart do I need the beams
3.How much coax between the 2 feed points do I need
4.And as they are horizontally polarized on a horizontal pole.
Would it throw the antenna pattern out unless i use a 2 vertical poles to rest it higher and out the way

Thanks
Will

Message from Calico at Saturday, 18-Sep-10 19:11:22 GMT

Hi Will,

Read about your nice effort at Global Tuners Forum, regarding stacking antennas, good luck.


I have been doing this sort of work with MF and HF antennas, not though for VHF as in your case. Some basic principles, I would suggest:

1) Both antenna coax feeders, cut from same length (or drum) of coax and if possibly exactly the same length. This is important as the time that the signal takes to travel down the coax harness needs to be as much as possible the same so that we have co-phasic signals. As usual use the formula for wavelength (in meters) derived from 300/f (in MHz) times velocity factor of your cable.

2) Connect both atnennas to an appropriate combiner, such as eBay item 150493024152.

3) Assuming that your antennas are for broadcast WFM reception, I would separate the antennas by 1/2 λ (lamda) and if I had the room, I would experiment varying the separation for the result that I was interested (phasing games) from 1/2 to 1/1 lamda.


4) I would try and maintain both antennas on the same vertical mast and at the same height. I hope you will have very interesting results, Will, phasing is very rewarding, and has many applications, from amplifying desired station beamings, to attenuating/nulling out unwanted locals, don't know what your priority is. For some sorts of phasing, feeders can be other lenghts too, but let's keep it simple for now.

In my case (MF, HF) , I rotate the phase (time) of the signals from each and every antenna and null out for example UK/European MW stations, so as to listen to USA MW stations, e.g. 1010 kHz (talkradionetwork) from the East Coast.


How are your studies / work going ?

73
Tim