Two identical antennas

[nighthawks]

Two identical antennas properly phased question.

Hello,
I just got these 2 yagi identical antennas, mounted them 1/4 wavelength apart.
I'm using 2 - 50 foot lengths RG6 into my 4 scanners.
With a metal box I made 2 BNC (in) and 4BNC (out) they are all grounded by the box and sodered all center conductors together, works well, no loss.

My question is are these antennas co-phased or do I need to do something different with the coax? I've done much reading but just not sure about co-phasing to identical receive antennas
Thanks John

You can find these here. Small antenna, big results. Then look for "items for sale"
http://myworld.ebay.com/bymajuro/

 

[photovision]

Here is a thought to start........

Usually when 2 antennas are co-phased, they are joined by a special coax set up and go to the same radio. For example, in CB the trucker mobile antennas are mounted on each side of the cab and both lead into a factory made coax " Y harness" and then one output goes into the CB radio. This is so the antennas are the correct wavelength apart so that the signal that they recieve is additive rather than 2 signals trying to cancel each other out. With all your conductors soldered together, it doesn't seem like they are actually co phased in the technical sense from what I can see. How do you know for sure that there is no loss? Did you take measurments or was it a by ear kind of determination? Also with 2 beam type antennas you would hope for a signal gain in the direction that each is pointing. Anyway, just some thoughts to get things started.

 

[jonny290]

This is an improperly wired setup. To 'co-phase' a directional array, they need to be pointing the same direction, have carefully measured spacing, and have a special coax harness wired up.

Your "2 in, 4 out" BNC box is just introducing phasing, loss, cross-mod and impedance mismatches.

Much better: A gain-producing (couple of 5/8 waves or something) vertical omni antenna. It looks like you're trying to 'listen far' in multiple directions - your best bet will be either an antenna rotor or a good vertical, maybe with preamp.

 

[LarrySC]

Are they both pointed in the same direction?? If so they must be .95 wavelengths apart if side by side or did you mount as shown in this STOCK photo. ??

 

[nighthawks]

Usually when 2 antennas are co-phased, they are joined by a special coax set up and go to the same radio. For example, in CB the trucker mobile antennas are mounted on each side of the cab and both lead into a factory made coax " Y harness" and then one output goes into the CB radio. This is so the antennas are the correct wavelength apart so that the signal that they recieve is additive rather than 2 signals trying to cancel each other out. With all your conductors soldered together, it doesn't seem like they are actually co phased in the technical sense from what I can see. How do you know for sure that there is no loss? Did you take measurments or was it a by ear kind of determination? Also with 2 beam type antennas you would hope for a signal gain in the direction that each is pointing. Anyway, just some thoughts to get things started.

I'm trying to figure out if the same co-phase applies for these rec. antennas.


Hooked up a regular splitter and then the one I made, more of a (ear) determination. locked in a station in Cleveland Ohio, 50 miles away, switch between splitters, the one I made had less hissing, more clear. I ran a diamond discone on a 50' tower with a pre amp for 20+ years, was happy with it, without the amp no Cleveland stations. But anyway sold my estate and moved to Tennessee. Now just messing around.
The person I got these from on eBay told me to mount them 1/4 wavelength apart point them in different directions, he had a ham guy build a co phased box with some kind of circuit board, soldered both coaxes coming in and then off the board one coax coming out to his scanner. He didn't have the diagram of this co-phased box, so I'm getting some ideas.
Thanks

 

[nighthawks]

This is an improperly wired setup. To 'co-phase' a directional array, they need to be pointing the same direction, have carefully measured spacing, and have a special coax harness wired up.

Your "2 in, 4 out" BNC box is just introducing phasing, loss, cross-mod and impedance mismatches.

Much better: A gain-producing (couple of 5/8 waves or something) vertical omni antenna. It looks like you're trying to 'listen far' in multiple directions - your best bet will be either an antenna rotor or a good vertical, maybe with preamp.

Well there seems to be many ways to do the co-phasing, I've been reading about.

The person I got these from on eBay told me to mount them 1/4 wavelength apart, point them in different directions, he had a ham guy build a co phased box with some kind of circuit board, soldered both coaxes coming in and then off the board one coax coming out to his scanner, he didn't explain this co-phasing box, but told me there were many way to do this.

With this setup now, I'm pulling in stations 60+ miles away, so something is working.
Thanks.

 

[nighthawks]

Are they both pointed in the same direction?? If so they must be .95 wavelengths apart if side by side or did you mount as shown in this STOCK photo. ??

Mount as shown in photo.

The person I got these from on eBay told me to mount them 1/4 wavelength apart, point them in different directions, he had a ham guy build a co phased box with some kind of circuit board, soldered both coaxes coming in and then off the board one coax coming out to his scanner

Regards, John

 

[prcguy]

The ad for these antennas on Ebay is very misleading and his explanations of radio reception are full of nonsense. The antenna appears to be a 3 element yagi cut for the UHF band or possibly 800MHz and the seller claims it works great from 150MHz to 800MHz. Can you measure the length of the elements and verify if the center element has the coax attached and if the other elements are floating in the plastic boom like a typical yagi director and reflector? That would tell us more of what you have. It would be typical not to receive the detailed info on co-phasing two of these from the seller as he is probably feeding more BS to sell more antennas. For one thing, co-phasing is limited to a reasonably narrow range of frequencies like 150-160MHz, not the entire range claimed by the seller. That’s my 2c, anyone else got a comment?
prcguy

 

[Raccon]

Pointing the antennas in different directions is ok if you want to achieve a different pattern than compared to one antenna alone. The gain will of course be lower as compared to pointing the antennas in the same direction. (The statements assume the use of a phasing harness and correct spacing.)

If mounted as shown in the picture you get a somewhat eliptical pattern (antennas in the center of it), but the use of a phasing harness is still recommended to maintain the correct impedance.

 

[jonny290]

prcguy is dead on, you need to have resonant coax lengths in the phasing harness, and you've got maybe 5, maybe 10% bandwidth deviation (e.g. 400-440 mhz or 800-880) tops before the setup starts to suck.

 

[ReceiverBeaver]

In an effort to sum it up briefly NH, what you have now is simply 2 separate antennas mounted on the same mast. The minimal 1/4 wave separation between is standard practice to keep two different antennas from interfering with each other's independant operation. So they are not cophased. But they should be...cophaseable....with a properly made-matched cophaseing harness connecting the two and bringing only 1 coax down to the shack. You can then configure the two antennas just like they are in your picture and they will provide an omindirectional pattern. Or you mount them on the same side of the mast one atop the other and achieve some useful gain in the direction they are pointing. Add a light duty TV rotor and you have a rotatable gain antenna.

This cophasing of yagi beams is usually seen with single band antennas where the elements are of more equal length. Your yagis apparently are log-periodic style with shorter-to-longer elements and covering more band segments. I don't see any reason why they wouldn't work just as well as the dipole-style single band yagis for stacking. Especially as you're receive-only here.

The Beaver says good luck to you, and may there be many choice munchies spread about on your holiday tables and the holiday tables of your friends and associates where you may chance to visit over the season. Munch well.

 

[nighthawks]

The ad for these antennas on Ebay is very misleading and his explanations of radio reception are full of nonsense. The antenna appears to be a 3 element yagi cut for the UHF band or possibly 800MHz and the seller claims it works great from 150MHz to 800MHz. Can you measure the length of the elements and verify if the center element has the coax attached and if the other elements are floating in the plastic boom like a typical yagi director and reflector? That would tell us more of what you have. It would be typical not to receive the detailed info on co-phasing two of these from the seller as he is probably feeding more BS to sell more antennas. For one thing, co-phasing is limited to a reasonably narrow range of frequencies like 150-160MHz, not the entire range claimed by the seller. That’s my 2c, anyone else got a comment?
prcguy

This is the website that's on the instructions.
http://www.centerfireantenna.com/70yagi.html
I don't know, just playing around with them, getting clear trans. up to 60+ miles away.
Thanks, John

 

[nighthawks]

Pointing the antennas in different directions is ok if you want to achieve a different pattern than compared to one antenna alone. The gain will of course be lower as compared to pointing the antennas in the same direction. (The statements assume the use of a phasing harness and correct spacing.)

If mounted as shown in the picture you get a somewhat eliptical pattern (antennas in the center of it), but the use of a phasing harness is still recommended to maintain the correct impedance.

This is the website on the instructions.
http://www.centerfireantenna.com/70yagi.html
Getting trans upto 60+ miles, seems to be working, just wondering if the co-phase would help even more.
Thanks John

 

[nighthawks]

prcguy is dead on, you need to have resonant coax lengths in the phasing harness, and you've got maybe 5, maybe 10% bandwidth deviation (e.g. 400-440 mhz or 800-880) tops before the setup starts to suck.

This is the website on the instructions.
http://www.centerfireantenna.com/70yagi.html
Getting trans upto 60+ miles, seems to be working, question is what is the co-phase lengths?

1 Lengths from each antenna. 50 ohm or 75?
2 Do I use a T connector and a barrel?
3 main feed line into my scanners 50 ohm or 75?
Thanks

 

[nighthawks]

The cophasing harness is what I'm trying to figure out, what freq. sould I use then do the math, is each leg from antenna 50 or 75 ohm, to the "T" connector/barrel, main line into shack 50 or 75 ohm?
Thanks

 

[N_Jay]

The cophasing harness is what I'm trying to figure out, what freq. sould I use then do the math, is each leg from antenna 50 or 75 ohm, to the "T" connector/barrel, main line into shack 50 or 75 ohm?
Thanks

You do the math for the freq you want.

At almost all other freqs they are not in phase.

 

[jonny290]

Phasing cables are calculated with two numbers: wavelength desired, and Velocity Factor (VF) of the cable. VF = how fast, relative to speed of light in a vacuum, the signal travels down the wire.

Common VFs of cables are 0.70 or thereabouts, depending on coax type.

Cable length = wavelength desired * velocity factor

From this, you can see that a phasing cable will always be *shorter* than the free-air wavelength. Then you start to realize that "hey, I can't put these antennas a half wave apart and use two quarter wavelength phasing cables (think about it), but I *can* use two 3/4 wave long phasing cables", which will give you ample wiring room and are more or less electrically identical to two 1/4 wave cables.

God, I love RF. Nigh on black magic, it is.

 

[N_Jay]

God, I love RF. Nigh on black magic, it is.

I love RF, but there is no black magic to it at all.

It is all science.

 

[prcguy]

I went to the centerfire site and aah, it’s a UHF yagi centered at 450MHz. Use around 450MHz as the design freq for your phasing harness, which would be made from 75 ohm coax. You will also have to space the antennas at the proper distance to achieve the maximum benefit, which is only 3dB gain. A single 5 element UHF yagi will give about the same results without all the hassle. Keep in mind that you have a UHF yagi made for about 420 to 470MHz and its not going to work as an antenna at VHF or 800MHz like the Ebay ad says. At these other frequencies it’s not an antenna, it’s just a random (and small) hunk of metal sticking in the air.
prcguy

 

[kf4lne]

I love RF, but there is no black magic to it at all.

It is all science.

SHUT UP! Don't tell anyone else! Let them all think its black magic and they have to come to a RF tech/witch doctor to get their radios to work! Don't let the masses find out theres not much more to it than math, theory and science and a little solder!