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Fw: Tuning a 2.4ghz feed without the dish?




----- Original Message -----
From: "Murray Peterson VK2KGM" <vk2kgm@ihug.com.au>
To: <nx7u@arrl.net>
Sent: Monday, April 29, 2002 4:14 PM
Subject: Re: [amsat-bb] Tuning a 2.4ghz feed without the dish?


| I have "played" with a dipole feed on a 600 mm grid dish where the focal
| point is 320 mm in front of the centre of the dish with the dipole
connected
| to a network analyser plotting reflected power.
|
| At 2.4 GHz only things that are within one wavelength have any measurable
| effect. The persence or absence of the dish makes no measurable difference
| ie less than -60db with respect to the input signal. After doing this
| myself, I am quite confident that the dish does not affect the tuning of
the
| dipole if it is more than a wavelength away which is almost all the time.
|
| This is my experience.
|
| regards,
| Murray Peterson
| VK2KGM
|
| ----- Original Message -----
| From: "Scott Townley" <nx7u@arrl.net>
| To: <g4klx@qsl.net>
| Cc: "Greg D." <ko6th_greg@hotmail.com>; <amsat-bb@AMSAT.Org>
| Sent: Monday, April 29, 2002 3:18 PM
| Subject: Re: [amsat-bb] Tuning a 2.4ghz feed without the dish?
|
|
| | Jonathan,
| | You are correct, in that my statement is not very well written.
| | *All* the energy illuminating the dish (in transmit mode) of course does
| | not return to the focus/dipole.  Some does, however, that being
| | reflected from the area immediately surrounding the vertex of the dish.
| | Depending upon the focal distance in wavelengths, this energy can be
| | either in-phase or out-of-phase...or in between...I don't think any
| | general statement can be made about its phase...but it's there in any
| | event.
| | Hence the "vertex plate" I mentioned.  Again, MIT Rad Lab Series, also
| | reprinted as "Microwave Antenna Design", by Silver.
| | I'm not sure what you mean by "an ideal dish would have a null at zero
| | degrees", so I'll skip that for now.  Nevertheless, there is a definite
| | interaction between the dipole and the dish.
| |
| | I don't disagree with the helix being low-Q.  It's just that I imagine
| | the *principle* reason that there  is less interaction is due to the
| | crosspolarization of the reflected energy near the dish vertex.  A
| | turnstile array (which I don't believe is low-Q) shows the same lack of
| | interaction.  Of course the linearly polarized dipole has no such
| | luxury.
| |
| | And whether a dish is transmitting or receiving, makes not a whit of
| | difference, impedance-wise.  But system performance wise, the receive
| | case for the typical amateur is the more critical one, again because of
| | the noise matching of the 1st LNA.
| |
| | And yes, offset dishes make this whole argument largely moot.  Wish I
| | had one...:-)
| |
| | Jonathan Naylor wrote:
| | >
| | > Hi Scott
| | >
| | > >  I don't think I can agree with most of that assessment.  But I'm
| | > > willing to learn if I'm off base.
| | >
| | > There appears to be a major inconsistency in your logic, and I have a
| | > couple of extra points to add.
| | >
| | > >  Dipole feeds are very much affected by the presence of the
| | > >  dish...remember that all that dish energy is focussed right back at
| | > > the dipole.
| | >
| | > If this were the case then no-one would use dishes. Think about the
| | > geometry of a dish, only RF that goes straight out from the feed into
a
| | > prime focus dish will come straight back, and probably out of phase.
| | > The rest of it will hit the dish elsewhere and be reflected out into
| | > space and do what the dish is supposed to do. An ideal dish feed would
| | > have a null at zero degrees.
| | >
| | > The helix does have a low Q (see any textbook) and as such is less
| | > affected by nearby objects, dishes included.
| | >
| | > My two extra points are related to the more specific example of the
| | > question asked, 2.4 GHz receive feed and dish, no transmitted RF
there.
| | > In this modern age offset fed dishes are very common and completely
| | > bypass any argument about feed blockage/RF feedback anyway.
| | >
| | > Jonathan  HB9FTF/G4KLX
| | ----
| | Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
| | To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
| |
|

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