[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Fw: Re: Re: Re: backfire helix question.



Tab matching of a monofilar axial mode helical feed is done as the S2 
feed on AO-40.
It has been clear ( Kraus ) that the helical coil can be modified many
ways and provide
interesting results.
However one of the criteria before matching to a transmission line in
order to obtain maximum 
gain was having the same phase shift between ALL turns.
Any other configuration provides changes to the max gain theory. ( my
understanding ) 
When however we leave the pure helix coil for a feed antenna function and
it's transmission 
line matching requirement , the phase shift between turns is, generally
speaking, is modified.
Maybe the solder on tapered shim stock impedance matching of the first
1/4 WL turn  
should start the turns count a the end of the shim stock.
I took a machined 3/4 WL reflector , attached the normal peripherally
mounted "N" connector 
and extended the center pin 1-1/2 inches ( 1.5 times the helical spacing
) above the reflector .
I proceed to determine how the VSWR reacted as I moved a unmodified coil
ever closer 
to the reflector.
The result was a gradual improvement in VSWR at each step.
The closer to the reflector the more improved the VSWR.
I wound up with 0.009 inch Teflon as the spacing between the beginning of
the coil ( to mainly prevent shorting ) and the reflector.
I then borrowed the Tab method of matching the transmission line.
The initial Tab is simply a piece of copper plate bent 90 degrees in the
middle.
Placing the L shaped Tab on the reflector in close proximity to the
beginning of the coil,
holding it with a finger and watching the results of RL , move the Tab
along the length of the 
coil until a good RL is obtained.    Maintaining  that position along the
length of the coil ,
the Tab is moved closer or further away from the coil to improve the RL.
This is a very sensitive but gross operation at this point .
The end result it to determine where along the length of the coil and
distance from the coil 
a very good impedance match can be had.
The interesting part of this method comes with wrap around Tab matching .
In order to have the Tab as far away from the coil as possible , the
length of the Tab 
and how the Tab forms to the curvature of the coil becomes important ( my
understanding)
At each step in the process I have gone through with my design ,
improvements have been 
valuable .
The one condition I have not tested and must be done is the measure of
axial ratio 
I believe that the ability to match the coil without changing the spacing
between turns 
has benefits.
I believe that the greater the number of turns relative to the beam width
required for a 
feed antenna the better the axial ratio.
I believe that changing the pitch in order to have greater number of
turns should be included.
I would like to know how the position of the phase center changes with
the use of Tab matching .
I found that the nice thing about the Tab was after attaching a 1/2 hard
line ( FL ) ( 5 feet long ) , moving the 
URL bridge to the end of the FL caused a slight change in URL which was
easily returned to it's original 
value by an ever so small adjustment of the Tab.
The Tab has proven to be a much easier matching method as long as you
have a URL bridge.
I use a 16 ua FS meter on my bridge.
I have the notion I could fashion a Tab match for your small backfire
reflector .
Whether that is true I'll have to wait till I get time to build the
backfire.
I have also been considering a machined version to your great efforts.

I hope I have not rambled to much.

Everytime I send a post I get a response from a server that indicates I
am a spam address.
Do I have to send this to the BB in order to reach you ?

Joe  K0VTY






 
   
 
On Tue, 18 Nov 2003 22:04:19 -0700 Scott Townley <nx7u@arrl.net> writes:
> OK.
> Well, I'll give it a run...generally line transformers are 
> semi-broadband, 
> but that of course depends upon the load.
> OK on stub, tab, etc...to me stub would have been a shunt coaxial 
> stub.  It 
> would have worked but the dimensional tolerances weren't possible 
> (for Me).
> If by "tab" you refer to the common practice of soldering a wider 
> strip of 
> metal to the 1st 1/4 turn of the helix...really that's just a series 
> line 
> section just like I've used with the backfire.  If you model the 
> input 
> impedance of an endfire helix, it comes up in the range of 100-150 
> ohms 
> real and -j50 ohms reactive or so.  A nearly-quarter-wave long, 120 
> ohm 
> line section would do the job (as a series line section)--and the 
> characteristic impedance of the oft-quoted 1/4" wide strip over a 
> ground 
> plane is on the order of 120 ohms!  In fact the U-band transmit 
> helix, 
> which doesn't have a ground plane at all, I matched using the 
> series-line 
> section idea, but with parallel/twin-conductor line instead.
> 
> At 14:04 2003-11-18, you wrote:
> >My attention to feed band width was created by the frequency
> >charactistics of the 1/4 WL stub.
> >A normal monofilar helical band width is wide .     How the 1/4 WL
> >matching section handles that wide band  width was what triggered 
> my
> >query.
> >The reference to Tab matching occurred when you commented :
> >"I considered stub and such when I realized that with a relatively 
> low
> >impedance , a single series-matching line transformer should be 
> able to
> >do the trick"
> >I incorrectly assumed your stub and tab were simular.
> >I have been successfully using Tab matching for helical feed 
> antennas.
> >If I had an address I could send pictures.    Your call.
> >
> >Joe  K0VTY
> >=============================
> >
> >On Tue, 18 Nov 2003 15:30:42 -0500 Scott Townley <nx7u@cox.net> 
> writes:
> > > I don't understand what you mean by "tab matching".
> > > And I don't know that it's narrowband...I simply measured the 
> RL
> > > from 2375 to 2425 because that's what I set the sweeper to :-) 
> I'll
> > > sweep it wider this afternoon and report.
> > > >
> > > > From: k0vty@juno.com
> > > > Date: 2003/11/18 Tue PM 01:32:52 EST
> > > > To: nx7u@arrl.net,  amsat-bb@AMSAT.Org
> > > > Subject: Re: [amsat-bb] Re: backfire helix question.
> > > >
> > > > Would your original thought of Tab matching remove the narrow
> > > banding ?
> > > >
> > > > Joe  K0VTY
> > > > ===============================
> > > > On Tue, 18 Nov 2003 13:10:22 -0500 Scott Townley 
> <nx7u@cox.net>
> > > writes:
> > > > > It's broader than 50MHz...I swept it from 2375 to 2425 and 
> it
> > > was
> > > > > <20dB RL the whole way...but I can open it up and report 
> back.
> > > > >
> > > > > >
> > > > > > From: k0vty@juno.com
> > > > > > Date: 2003/11/18 Tue AM 11:39:43 EST
> > > > > > To: nx7u@arrl.net,  amsat-bb@AMSAT.Org
> > > > > > Subject: [amsat-bb] Re: backfire helix question.
> > > > > >
> > > > > > Have you had a chance to look at the frequency band width 
> of
> > > > > > the backfire helical feed as you have configured the 
> matching
> > > ?
> > > > > >
> > > > > > Joe  K0VTY
> > > > > >
> > > > > >
> > > 
> ________________________________________________________________
> > > > > > The best thing to hit the internet in years - Juno 
> SpeedBand!
> > > > > > Surf the web up to FIVE TIMES FASTER!
> > > > > > Only $14.95/ month - visit www.juno.com to sign up today!
> > > > > > ----
> > > > > > Sent via amsat-bb@amsat.org. Opinions expressed are those 
> of
> > > the
> > > > > author.
> > > > > > Not an AMSAT member? Join now to support the amateur 
> satellite
> > >
> > > > > program!
> > > > > > To unsubscribe, send "unsubscribe amsat-bb" to
> > > > > Majordomo@amsat.org
> > > > > >
> > > > >
> > > > > Scott Townley NX7U
> > > > > Gilbert, AZ  DM43di
> > > > >
> > > > >
> > > > >
> > > >
> > > > 
> ________________________________________________________________
> > > > The best thing to hit the internet in years - Juno SpeedBand!
> > > > Surf the web up to FIVE TIMES FASTER!
> > > > Only $14.95/ month - visit www.juno.com to sign up today!
> > > > ----
> > > > Sent via amsat-bb@amsat.org. Opinions expressed are those of 
> the
> > > author.
> > > > Not an AMSAT member? Join now to support the amateur 
> satellite
> > > program!
> > > > To unsubscribe, send "unsubscribe amsat-bb" to
> > > Majordomo@amsat.org
> > > >
> > >
> > > Scott Townley NX7U
> > > Gilbert, AZ  DM43di
> > >
> > >
> > >
> >
> >________________________________________________________________
> >The best thing to hit the internet in years - Juno SpeedBand!
> >Surf the web up to FIVE TIMES FASTER!
> >Only $14.95/ month - visit www.juno.com to sign up today!
> >----
> >Sent via amsat-bb@amsat.org. Opinions expressed are those of the 
> author.
> >Not an AMSAT member? Join now to support the amateur satellite 
> program!
> >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> 
> Scott Townley NX7U
> Gilbert, AZ  DM43di
> http://members.cox.net/nx7u
> 
> 
> 

________________________________________________________________
The best thing to hit the internet in years - Juno SpeedBand!
Surf the web up to FIVE TIMES FASTER!
Only $14.95/ month - visit www.juno.com to sign up today!
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home