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 get 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 the matching and feedpoint of the driven element. Again those of you familiar with the WA5VJB " cheap yagi" concept will know that the driven element is 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 something going on right there. Either with the spacing between the legs of the U or 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 help 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 sense. At any rate I have decided to rebuild both my 70cm and 2 meter antenna. I 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 PVC 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 out velocity factor on cable while making phasing harnesses. Maybe someday! Thanks much for everyones help and 73, Michael W4HIJ
----- Original Message ----- From: "Michael Tondee" [email protected] To: "amsat-bb" [email protected] 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 get 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 the matching and feedpoint of the driven element. Again those of you familiar with the WA5VJB " cheap yagi" concept will know that the driven element is 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 something going on right there. Either with the spacing between the legs of the U or 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 help 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. I
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 PVC 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 out 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
Thank you Domenico, I've actually studied all those links before. I would build a CP antenna as Howard, G6LVB has done except that I have nothing with which to measure the 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 the LEO's. Unfortunately I have no experience with HEO's. 73, Michael, W4HIJ ----- Original Message ----- From: "i8cvs" [email protected] To: "Michael Tondee" [email protected]; "amsat-bb" [email protected] Sent: Thursday, June 05, 2008 8:46 PM Subject: Re: [amsat-bb] 70cm antenna SWR problem solved?
----- Original Message ----- From: "Michael Tondee" [email protected] To: "amsat-bb" [email protected] 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 get 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 the matching and feedpoint of the driven element. Again those of you familiar with the WA5VJB " cheap yagi" concept will know that the driven element is 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 something going on right there. Either with the spacing between the legs of the U or 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 help 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. I
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 PVC 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 out 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
--------------------------------------------------------------------------------
No virus found in this incoming message. Checked by AVG. Version: 8.0.100 / Virus Database: 270.0.0/1487 - Release Date: 6/6/2008 8:01 AM
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.
Have fun
73" de
i8CVS Domenico
----- Original Message ----- From: "Michael Tondee" [email protected] To: "i8cvs" [email protected]; "amsat-bb" [email protected] 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
as
Howard, G6LVB has done except that I have nothing with which to measure
the
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
the
LEO's. Unfortunately I have no experience with HEO's. 73, Michael, W4HIJ ----- Original Message ----- From: "i8cvs" [email protected] To: "Michael Tondee" [email protected]; "amsat-bb" [email protected] Sent: Thursday, June 05, 2008 8:46 PM Subject: Re: [amsat-bb] 70cm antenna SWR problem solved?
----- Original Message ----- From: "Michael Tondee" [email protected] To: "amsat-bb" [email protected] 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
get
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
the
matching and feedpoint of the driven element. Again those of you
familiar
with the WA5VJB " cheap yagi" concept will know that the driven element
is
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
something
going on right there. Either with the spacing between the legs of the U
or
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
help
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.
I
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
PVC
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
out
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
------
No virus found in this incoming message. Checked by AVG. Version: 8.0.100 / Virus Database: 270.0.0/1487 - Release Date: 6/6/2008 8:01 AM
Yes, I know the Bird 43 well. Years ago I worked as a shipping clerk in a ham radio store and I remember that we sold them as well as the fact that we had one on our test bench. I now regret not getting one back then when I had my employee discount. As it was, I was like the proverbial "kid in a candy store" and was lucky to get home with my paychecks intact.... hi hi Michael ----- Original Message ----- From: "i8cvs" [email protected] To: "AMSAT-BB" [email protected]; "Michael Tondee" [email protected] Sent: Friday, June 06, 2008 11:48 PM Subject: Re: [amsat-bb] 70cm antenna SWR problem solved?
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.
Have fun
73" de
i8CVS Domenico
----- Original Message ----- From: "Michael Tondee" [email protected] To: "i8cvs" [email protected]; "amsat-bb" [email protected] 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
as
Howard, G6LVB has done except that I have nothing with which to measure
the
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
the
LEO's. Unfortunately I have no experience with HEO's. 73, Michael, W4HIJ
Michael,
I have not had good luck with Marine Spar Varnish on my WA5VJB cheap yagis. Even if you use high quality spar varnish, it is damaged with the constant exposure to sunlight. Signs of damage were apparent after one year and almost total loss after the second year. My antennas were exposed to the NY weather which I would think is not as bad as yours with regard to the sunlight.
My portable satellite station uses varnish coated WA5VJB antennas, but they spend most of their time stored in my garage or otherwise protected from the elements. They are better than 5 years old and still have a nice gloss finish.
I am trying high quality exterior house paint this time
73 de WB2LLP Gene
participants (3)
-
i8cvs -
Michael Tondee -
WB2LLP