[amsat-bb] Re: 70cm antenna SWR problem solved?
domenico.i8cvs at tin.it
Fri Jun 6 20:48:22 PDT 2008
Hi Michael, W4HIJ
For the velocity factor use the value given by the manufacturers for each
type of coax cable.
As an example a Vf= 0.66 is OK for the RG 213 and RG-58
Don't make long run of coax without the braid in the vicinnity of the feed
point because the inner conductor of the coax cable will add to the lenght
of the dipole.
When you measure the SWR slide the palm of your hand over the jacket of the
coax cable and if the SWR changes it means that part of the RF current is
flowing over the external braid of the coax cable and so you need a balun.
To measure the incident and reflected power to compute the SWR try to buy
a BIRD 43 wich is a professional instrument and buy time to time the
measuring elements to satisfy your needs.
In a couple of years you will get the most practical and versatile
instrument for RF measurements in the shack and over the tower.
----- Original Message -----
From: "Michael Tondee" <mat_62 at netcommander.com>
To: "i8cvs" <domenico.i8cvs at tin.it>; "amsat-bb" <amsat-bb at amsat.org>
Sent: Friday, June 06, 2008 9:47 PM
Subject: Re: [amsat-bb] 70cm antenna SWR problem solved?
> Thank you Domenico,
> I've actually studied all those links before. I would build a CP antenna
> Howard, G6LVB has done except that I have nothing with which to measure
> velocity factor of the cable in the phasing harness. When it was working
> the 70cm linearly polarized did a pretty good job though. At least with
> LEO's. Unfortunately I have no experience with HEO's.
> Michael, W4HIJ
> ----- Original Message -----
> From: "i8cvs" <domenico.i8cvs at tin.it>
> To: "Michael Tondee" <mat_62 at netcommander.com>; "amsat-bb"
> <amsat-bb at amsat.org>
> Sent: Thursday, June 05, 2008 8:46 PM
> Subject: Re: [amsat-bb] 70cm antenna SWR problem solved?
> > ----- Original Message -----
> > From: "Michael Tondee" <mat_62 at netcommander.com>
> > To: "amsat-bb" <amsat-bb at amsat.org>
> > Sent: Friday, June 06, 2008 1:30 AM
> > Subject: [amsat-bb] 70cm antenna SWR problem solved?
> >> After several unsuccessful attempts at solving my 70cm antenna problem
> > while it was still on the mast, I resolved today to just go ahead and
> > it
> > down off there. Not that big a deal to do really and besides I've been
> > thinking of rebuilding the whole shootin match anyway.
> >> I was able to use my 2 meter SWR meter after a fashion once I got the
> > antenna inside. I could see fluctuations in the amount of power the
> > antenna
> > was taking as I tried various things. The problem lies in the area of
> > matching and feedpoint of the driven element. Again those of you
> > with the WA5VJB " cheap yagi" concept will know that the driven element
> > bent back upon itself forming a U shaped section on one side. Those less
> > familiar with the design might not understand this but there's
> > going on right there. Either with the spacing between the legs of the U
> > the braid and center conductor being too close together at that point
> > where
> > they are soldered. Changing the spacing of the U legs didn't seem to
> > but spreading the center conductor and shield out from each other right
> > where they are connected to the DE made a marked difference. I suspect
> > it's
> > because at this point they are very close to the "matching area" of the
> > antenna. Hope that makes se!
> >> nse.
> >> At any rate I have decided to rebuild both my 70cm and 2 meter antenna.
> > will keep 11 elements on 70cm but I'm going from 4 to 6 elements on 2
> > meters. The other change will be to replace the boom material. I used
> > pipe before and the antennas have developed a bit of a sag along their
> > booms. I will be using wooden booms coated with spar varnish this go
> > around
> > and possibly add reinforcement if any sag appears evident. BTW, both of
> > these are linearly polarized as I don't have an SWR analyzer to figure
> > velocity factor on cable while making phasing harnesses. Maybe someday!
> >> Thanks much for everyones help and 73,
> >> Michael W4HIJ
> > Hi Michael, W4HIJ
> > Before to rebuild your WA5VJB cheap yagi antenna look here:
> > http://www.qsl.net/ve3cvg/antennas/cheapyagis/index.html
> > here
> > http://webpages.charter.net/w7my/cheap%20yagis%20designuhf-sw.pdf
> > and here:
> > http://www.g6lvb.com/HomebrewArrow.htm
> > 73" de
> > i8CVS Domenico
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