I may have misunderstood but the way I got it when I was researching homebrewing yagi's for my sat station is that if you had to fix the antenna to either RHCP or LHCP and could not switch between the two because of cost or complexity or whatever other reason that your best compromise would be to go with linear polarization. Feed harnesses for homebrew CP antennas are a stumbling block for me as I don't have a grid dip meter or SWR anylyzer to properly figure coax velocity and electrical length. If you don't get the phasing harnesses the right length then you won't get CP anyway. I use two of the "cheap yagi" designs by Kent Britain in vertical polarization and rotate them with a homebrew "SAEBRTrack" Az/El rotor box and old Gemini OR-360 TV rotators. Works well enough for LEO's anyway. 73, Michael, W4HIJ

Hi Ed, I have The Satellite Experimenters Handbook, it was one of my very first purchases when I decided to get into this. Actually that is where I got the impression that velocity factor and electrical length of the phasing harnesses was a big deal. Davidoff states that there can be a 10% variation off published values from cable to cable. That is what I. thought prevented me from building a phasing harness without proper measuring equipment. If I can get "close enough" without a grid dip meter ot SWR anylyzer, that would be great. I actually already have a second set of elements cut for my 70cm antenna and I purposely left the boom long enough to accomodate them. At any rate I wish I did have an SWR anylyzer that would cover 2 meters and 70cm but my hobby budget won't allow it at this time. I have a garden variety 2 meter SWR bridge but nothing that works on 70 cm which is a problem for me because I'm having trouble with the TX on the 70cm uplinks of VO-29 and AO-7 and I suspect it's SWR related. 73, Michael, W4HIJ ----- Original Message ----- From: "Edward Cole" kl7uw@acsalaska.net To: "Michael Tondee" mat_62@netcommander.com Cc: amsat-bb@amsat.org Sent: Saturday, September 20, 2008 1:59 AM Subject: Re: [amsat-bb] Re: Polarity questions

Michael:

At 05:53 PM 9/19/2008, Michael Tondee wrote:

...

I may have misunderstood but the way I got it when I was researching homebrewing yagi's for my sat station is that if you had to fix the antenna to either RHCP or LHCP and could not switch between the two because of cost or complexity or whatever other reason that your best compromise would be to go with linear polarization.

It might be easier to wire for just one sense of CP (maybe a little cheaper), but the complexity is not that big a deal. You can read how to do it in the Satellite Experimenter's Handbook by Davidoff (available from Amsat or ARRL).

...

Feed harnesses for homebrew CP antennas are a stumbling block for me as I don't have a grid dip meter or SWR anylyzer to properly figure coax velocity and electrical length. If you don't get the phasing harnesses the right length then you won't get CP anyway.

You are making too much of a big deal out of this. Velocity factor is published by coax manufacturers so you do not need instruments to get close. 1/4 WL = Vf * 492/Fmhz in feet 1/4 WL (RG-213) = 0.66 * 492/144 = 2.25 feet or 27-inches

If you are off 5% it will not destroy the circularity. Most hams get it "close enough". Of course if you are off by a quarter wavelength or more it will matter (20-inches at 144-MHz). I know you can get it within an inch and it will work fine. More accuracy produces a better SWR.

If you plan to build much VHF and above stuff a SWR meter is a good basic piece of equipment to have.

...

I use two of the "cheap yagi" designs by Kent Britain in vertical polarization and rotate them with a homebrew "SAEBRTrack" Az/El rotor box and old Gemini OR-360 TV rotators. Works well enough for LEO's anyway. 73, Michael, W4HIJ

Sure, linear antennas only sacrifice 3-dB of gain in crosspolarization with a CP signal. Often squint angle make such signals elliptical so the loss can be less.

GL

---

73, Ed - KL7UW BP40iq, 6m - 3cm 144-EME: FT-847, mgf-1801, 4x-xp20, 185w http://www.kl7uw.com AK VHF-Up Group NA Rep. for DUBUS: dubususa@hotmail.com

---

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Hmmmmm might get shot down in flames for this but.

If you want to know the velocity factor of a coax (known or un-known) try getting two eaqual lengths of coax which are physically 1/4wavelength long i.e. 50cms for 2mtrs. Short out ONE end (join the inner and outers together with a short a link as possible) and do this for both pieces of coax. We are going to make a coaxial dipole so these two shortened ends go on to a piece of feeder (try and keep this feeder as short as possible also but the dipole needs to be somewhere clear but does not to be 100 feet up either). Yes you will need to support the dipole elements as they will just hang down so perhaps a piece of garden cane or some other RF invisible material. Trim back both OPEN ends of the dipole until you get the lowest possible VSWR (on the centre frequency you want to use) and you should have very close to an electrically 1/2wave dipole for the coax you used (each side of the dipole will be a 1/4 wave long). You can use these figures for making a 1/4wave delay line or if your making a polarity control box you will need to know the lengths of both 1/2 and 1/4 of the coax you are going to use, the dipole will be considerably shorter than a normal wire dipole for the same frequency because we have used the velocity factor to help reduce its length.

If you think that the feeder you have used is radiating either do perhaps 6 turns around a small 1" tube to make a balun at the feedpoint or alter the length of the feeder slightly, if theVSWR alters when you change the length of the feeder then the feeder is radiating.

My two pennys worth, should help you get very close to the figures you need without the use of a GDO or an antenna analyser.

regards Gus

Hi Gus

To know the velocity factor of a coax (known or un-known ) with good accuracy I make the measurement at about 30 MHz because the velocity factor do not change with frequency and a low frequency measurement make the error the smallest as possible.

Get a N coax T and connect the male port of it to the vertical Y channel of an oscilloscope.

A spectrum analyser is not necessary because any oscilloscope works at 30 MHz

Connect one female port of the T connector to a RF generator and set it to 30 MHz

If you don't have a RF signal generator use a low power 30 MHz TX at 1 or 2 watt output power or less using few suitable 50 ohm attenuators.

Connect the other female port of the T connector to an estimated 1/4 electrical wave long of coax cable under test and live open the other end of it.

______________ _______________ | | | 30 MHz | | Oscilloscope | | signal generator | | | | or low power TX | |___________Y_ | |_______________| 1/4 Lambda /| / open o------------|------------------------| end T connector

If you suspect that the velocity factor is 0.66 like for solid polyethylene (PE) make the 1/4 electrical wave at 30 MHz 1.65 meters i.e. 65" long

The measurement plane of the coax cable under test is the middle of the T connector just between both female ports where the generator and the coax under test get in contact each other so that the real lenght of 1/4 electrical lenght must be taken from the above measurement plane including the male N connector up to the open end of it.

Because of the theory of transmission line and following the Smith Chart we see that a 1/4 electrical wave long transmission line becames a short circuit at his resonating frequency when the other end of it is open.

While looking at the CRT of the oscilloscope adjust the frequency of the signal generator until the amplitude of the signal on the CRT falls abruptly to zero.

The frequency shown by the signal generator is the resonant frequency of the 1/4 electrical wave.

Knowing the frequency and the physical lenght of 1/4 wave it is easy to calculate the real velocity factor of the coax cable under test.

No special instruments are necessary because about everyone own a signal generator for 30 MHz and a CRT oscilloscope.

Using a GDO or an antenna analyser the measurement can be wrong because it is difficult to know if the above instruments made for amateur use are well calibrated or not.

73" de

i8CVS Domenico

----- Original Message ----- From: "Angus" angus@young5769.freeserve.co.uk To: AMSAT-BB@amsat.org Sent: Sunday, September 21, 2008 7:57 AM Subject: [amsat-bb] Re: Polarity questions

Hmmmmm might get shot down in flames for this but.

If you want to know the velocity factor of a coax (known or un-known) try getting two eaqual lengths of coax which are physically 1/4wavelength long i.e. 50cms for 2mtrs. Short out ONE end (join the inner and outers together with a short a link

as

possible) and do this for both pieces of coax. We are going to make a coaxial dipole so these two shortened ends go on to

a

piece of feeder (try and keep this feeder as short as possible also but

the

dipole needs to be somewhere clear but does not to be 100 feet up either). Yes you will need to support the dipole elements as they will just hang

down

so perhaps a piece of garden cane or some other RF invisible material. Trim back both OPEN ends of the dipole until you get the lowest possible VSWR (on the centre frequency you want to use) and you should have very close to an electrically 1/2wave dipole for the coax you used (each side

of

the dipole will be a 1/4 wave long). You can use these figures for making

a

1/4wave delay line or if your making a polarity control box you will need

to

know the lengths of both 1/2 and 1/4 of the coax you are going to use, the dipole will be considerably shorter than a normal wire dipole for the same frequency because we have used the velocity factor to help reduce its length.

If you think that the feeder you have used is radiating either do perhaps

6

turns around a small 1" tube to make a balun at the feedpoint or alter the length of the feeder slightly, if theVSWR alters when you change the

length

of the feeder then the feeder is radiating.

My two pennys worth, should help you get very close to the figures you

need

without the use of a GDO or an antenna analyser.

regards Gus

-- I am using the free version of SPAMfighter for home users. SPAMfighter has removed 3111 spam emails to date. Paying users do not have this message in their emails. Get the free SPAMfighter here: http://www.spamfighter.com/len

---

Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

Hi Joe WB9SBD

At the beginning of TV the television set were feed with a 300 ohm twin lead.

Since the theory of transmission lines is not changed one method to eliminate TVI was to connect in parallel to the above feed line a 1/4 electrical wave of 300 ohm twin lead cut for the interfering frequency and with the other end open.

At the interfering frequency the above resonant 1/4 wave transmission line with open end represented a short circuit at the interfering frequency across the input of the television set.

Another method was to connect the same way a 1/2 electrical wave of 300 ohm twin lead cut for the interfering frequency but with the other end short circuited.

Since a 1/2 wave represents a full turn around the Smith Chart then the short circuit made at the other end was seen as a short circuit for the interfering frequency just at the input of the television set.

If you like to use the above setup for 300 ohm twin lead then you must use a balun to connect the unbalanced coax cable coming from the signal generator to a balanced transmission line as a 300 ohm twin lead.

Best 73" de

i8CVS Domenico ----- Original Message ----- From: Joe To: i8cvs Cc: Angus ; AMSAT-BB Sent: Monday, September 22, 2008 2:19 AM Subject: Re: [amsat-bb] Re: Polarity questions

Howdy all,

I Have a question,,

Will the below set up work with 300 ohm TV type of twin lead?

Joe WB9SBD

i8cvs wrote:

Hi Gus

To know the velocity factor of a coax (known or un-known ) with good accuracy I make the measurement at about 30 MHz because the velocity factor do not change with frequency and a low frequency measurement make the error the smallest as possible.

Get a N coax T and connect the male port of it to the vertical Y channel of an oscilloscope.

A spectrum analyser is not necessary because any oscilloscope works at 30 MHz

Connect one female port of the T connector to a RF generator and set it to 30 MHz

If you don't have a RF signal generator use a low power 30 MHz TX at 1 or 2 watt output power or less using few suitable 50 ohm attenuators.

Connect the other female port of the T connector to an estimated 1/4 electrical wave long of coax cable under test and live open the other end of it.

______________ _______________ | | | 30 MHz | | Oscilloscope | | signal generator | | | | or low power TX | |___________Y_ | |_______________| 1/4 Lambda /| / open o------------|------------------------| end T connector

If you suspect that the velocity factor is 0.66 like for solid polyethylene (PE) make the 1/4 electrical wave at 30 MHz 1.65 meters i.e. 65" long

The measurement plane of the coax cable under test is the middle of the T connector just between both female ports where the generator and the coax under test get in contact each other so that the real lenght of 1/4 electrical lenght must be taken from the above measurement plane including the male N connector up to the open end of it.

Because of the theory of transmission line and following the Smith Chart we see that a 1/4 electrical wave long transmission line becames a short circuit at his resonating frequency when the other end of it is open.

While looking at the CRT of the oscilloscope adjust the frequency of the signal generator until the amplitude of the signal on the CRT falls abruptly to zero.

The frequency shown by the signal generator is the resonant frequency of the 1/4 electrical wave.

Knowing the frequency and the physical lenght of 1/4 wave it is easy to calculate the real velocity factor of the coax cable under test.

No special instruments are necessary because about everyone own a signal generator for 30 MHz and a CRT oscilloscope.

Using a GDO or an antenna analyser the measurement can be wrong because it is difficult to know if the above instruments made for amateur use are well calibrated or not.

73" de

i8CVS Domenico

----- Original Message ----- From: "Angus" angus@young5769.freeserve.co.uk To: AMSAT-BB@amsat.org Sent: Sunday, September 21, 2008 7:57 AM Subject: [amsat-bb] Re: Polarity questions

Hmmmmm might get shot down in flames for this but.

If you want to know the velocity factor of a coax (known or un-known) try getting two eaqual lengths of coax which are physically 1/4wavelength long i.e. 50cms for 2mtrs. Short out ONE end (join the inner and outers together with a short a link

as

possible) and do this for both pieces of coax. We are going to make a coaxial dipole so these two shortened ends go on to

a

piece of feeder (try and keep this feeder as short as possible also but

the

dipole needs to be somewhere clear but does not to be 100 feet up either). Yes you will need to support the dipole elements as they will just hang

down

so perhaps a piece of garden cane or some other RF invisible material. Trim back both OPEN ends of the dipole until you get the lowest possible VSWR (on the centre frequency you want to use) and you should have very close to an electrically 1/2wave dipole for the coax you used (each side

of

the dipole will be a 1/4 wave long). You can use these figures for making

a

1/4wave delay line or if your making a polarity control box you will need

to

know the lengths of both 1/2 and 1/4 of the coax you are going to use, the dipole will be considerably shorter than a normal wire dipole for the same frequency because we have used the velocity factor to help reduce its length.

If you think that the feeder you have used is radiating either do perhaps

6

turns around a small 1" tube to make a balun at the feedpoint or alter the length of the feeder slightly, if theVSWR alters when you change the

length

of the feeder then the feeder is radiating.

My two pennys worth, should help you get very close to the figures you

need

without the use of a GDO or an antenna analyser.

regards Gus

-- I am using the free version of SPAMfighter for home users. SPAMfighter has removed 3111 spam emails to date. Paying users do not have this message in their emails. Get the free SPAMfighter here: http://www.spamfighter.com/len

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At 03:56 AM 9/20/2008, Anthony Monteiro wrote:

At 07:29 PM 9/19/2008, Clint Bradford wrote:

...

...

...

...I have to disagree with Clint...

I just must be the luckiest S.O.B.(*) working AO-51, then...During demonstrations in front of folks, someone in the assemblage almost always asks about polarization. I twist the Arrow, and we all "hear" no difference in its strong, clean signal.

Clint, K6LCS 909-241-7666

Dear Clint and friends,

There is no luck involved. The UHF downlinks on AO-51 are circularly polarized. You can twist your Arrow all you want and you will not hear any difference, nor could you measure any because there isn't any difference!

The cross-polarization loss from a circular TX antenna to a linear RX antenna is a constant 3 dB regardless of the actual linear polarization angle.

If you want to demonstrate polarization mismatch loss, use AO-27 (which is linear) with an Arrow.

73, Tony AA2TX

---

Well, theoretically the signal from AO-51 might be somewhat elliptical if there is a significant squint angle. Apparently this does not happen with enough change to be detectable. The human ear cannot detect audio level differences under 3-dB. Other effects probably mask any measurable variance.

73 Ed - KL7UW