MT magazine UHF Satcom antenna project

[Napalm]

Thanks, I appreciate you taking the time to reply.

In free space with no ground plane, there's barely any signal, but when mounted on a mag mount in the middle of the car roof the signals increase from barely anything to around and S2/3 on my MVT-7100 (I notice you have one too, so you know how hot they are at Milair). The bottom of the PVC support pipe is exactly 20 inches from the dipoles, and the magmount puts it around 22/23 inches. This seems to be the best result I can get, although anecdotally it seems like I have _some_ directivity.

It seems like a decent antenna though, and I am happy with it considering how much I spent, although my very homebrew three element Yagi gets better results, it is a pain to have to rotate it

I will keep trying to test the X-wing fully though, and my latest problem is the feeder radiates a little - I am using a length of 75R low loss coax from the Tee-piece to my scanner, and depending on where the feeder is on the car roof ,the signal comes or goes. I will invest in an F-to-N and use some 50R RG-213 or similar instead, thence to my preamp.

Thanks again.

There should be a huge difference in signals if the phasing harness is reversed and the bigger the difference the better meaning the phasing harness and element tuning is right on.

Reversing the connectors at the dipoles will reverse the polarization.

The antenna will work better in free space or way above a ground plane if its reversed but not nearly as good as with the ground plane and correct polarization. The dipole distance above the ground plane determines the pattern and whatever the article dimensions are will give an upward lobe that favors around 20 to 6o deg above the horizon.

If you raise the elements slightly above the groundplane (if assembled per spec) it will lower the lobes a little but they already sit close to the lowest angle you are likely to get. Building the antenna per spec and tilting the antenna a little is probably better for your sub 20deg look angle.
prcguy

 

[Napalm]

Just another update.

Antenna on the car roof, getting most of the LANT birds at around an S2/3. However, I rotated the antenna and apparently all is not well, because there's a definite null/peak situation going on.

What could cause this? My elements aren't all _exactly_ 10 inches - I never was too good at using the hacksaw. I reckon this can be the only cause. Everything is wired up right, and the phasing harness is, as far as I can tell, exactly the right length, tip to tip.

 

[prcguy]

It sounds like you have the phasing harness connected to the right ports.

I'm surprised you see any feedline influence and I considered using a string of #43 ferrite beads on each phasing line to decouple the dipole elements but that would be one more hurdle for the builder. You can add a string of beads and about 10 is the usual recommendation for VHF/UHF range.

You can also find the height for best reception between whatever satellites you can receive then make the PVC support mast that length. Moving the dipoles a little closer to the ground plane will raise the pattern up and raising them will lower the pattern. There will be a point where the dipoles and harness get detuned and I'm not sure how far you can go but +/- 2 or 3 inches should not cause a big problem and that might move the pattern up or down 5 degrees.
prcguy

Thanks, I appreciate you taking the time to reply.

In free space with no ground plane, there's barely any signal, but when mounted on a mag mount in the middle of the car roof the signals increase from barely anything to around and S2/3 on my MVT-7100 (I notice you have one too, so you know how hot they are at Milair). The bottom of the PVC support pipe is exactly 20 inches from the dipoles, and the magmount puts it around 22/23 inches. This seems to be the best result I can get, although anecdotally it seems like I have _some_ directivity.

It seems like a decent antenna though, and I am happy with it considering how much I spent, although my very homebrew three element Yagi gets better results, it is a pain to have to rotate it

I will keep trying to test the X-wing fully though, and my latest problem is the feeder radiates a little - I am using a length of 75R low loss coax from the Tee-piece to my scanner, and depending on where the feeder is on the car roof ,the signal comes or goes. I will invest in an F-to-N and use some 50R RG-213 or similar instead, thence to my preamp.

Thanks again.

 

[Napalm]

It sounds like you have the phasing harness connected to the right ports.

I'm surprised you see any feedline influence and I considered using a string of #43 ferrite beads on each phasing line to decouple the dipole elements but that would be one more hurdle for the builder. You can add a string of beads and about 10 is the usual recommendation for VHF/UHF range.

You can also find the height for best reception between whatever satellites you can receive then make the PVC support mast that length. Moving the dipoles a little closer to the ground plane will raise the pattern up and raising them will lower the pattern. There will be a point where the dipoles and harness get detuned and I'm not sure how far you can go but +/- 2 or 3 inches should not cause a big problem and that might move the pattern up or down 5 degrees.
prcguy

Right now, the dipoles are roughly 22/23 inches above the reflector (car roof) and that _seems_ to be the sweet spot.

I had a clip on ferrite in the junk box which I snapped on just before the coax leaves the support tube, and that has fixed my radiating feeder.

It's a good antenna and I want to persevere with it, I'm just not sure why I'm seeing slight nulls when its rotated.

 

[prcguy]

If you rotate the antenna in place and the signal strength changes that's not a good sign. I don't have the EZNEC plots of the antenna on this computer but I remember they were smooth with very little ripple in the upward pattern.

One measurement of how good the circular polarity is on an antenna is to measure it against a linear polarity antenna and rotate either antenna. The resulting change is called Axial Ratio and a perfect circular pol antenna would have 0dB change when rotated. A perfect circular pol antenna would also have a large difference between right hand and left hand pol on the order of 30dB or more.

The antennas on the satellite are very good and a satellite hanging in space makes for an ideal antenna test range with little if any influence from ground bounce and other reflections. I suspect your phasing harness is not perfect and your antenna is somewhat elliptical.

There is also a 3dB loss in signal when going between a circular pol and linear pol antenna but even a circular pol antenna with lousy axial ratio will get you most of that 3dB back at the expense of a little change when you rotate it.

I would suggest making a pair of test cables for the phasing harness and rotate the antenna to see which direction the phasing harness might need to go. Pick either the long or short cable and make one cable maybe in inch longer and one an inch shorter then substitute and test. You only need to concentrate on one of the cables to change the overall harness phase delay.

When the harness is optimized you will have the least amount of change when you rotate the antenna and also the most amount of change when you swap the phasing harness at the base of the dipoles.

Using a preamp and spectrum analyzer I could see about 10dB of change when swapping the phasing harness on my two prototype antennas and my measurement range was limited to the signals dropping into the noise floor. I don't know the ultimate circular cross pol or axial ratio, but its not bad for a home made project.
prcguy

 

[Napalm]

Right, I'll make something up tonight. Like I said, tip to tip, the long cable is within a millimetre or two of 13.5 inches. I can only assume that my tee-piece isn't quite right, or something else is throwing it out of whack.

I'll make a long cable of say, 13 inches, and one of 14 and see how it works.

It's a pain swapping cables though because my support tube is only 55mm in diameter (sorry, not sure what that is in inches) so getting fingers in there(!) to change things around is difficult.

If I have the theory right, the short cable is basically a continuation of the feeder, and the long cable is what sets up the delay - or is there some more matching going on in the 3.5 inch cable...? A fellow constructor on another forum made his antenna with the feeder going to one set of dipoles, and having a 10 inch length of RG-6 to the other set, and it works all right, from his tests anyway.

I understand why you used F connectors and tee-pieces - its easier (or in thereoy should be) for readers to reproduce the antenna if they're not messing about with soldering bare wires to each other, and I do take your point about the lack of braid on the RG-6.

Will report back when I've had chance to make up some alternate phasing harnesses.

David.

 

[prcguy]

The short cable is simply a convieniant length jumper to make it easier to screw the F connectors up inside the pipe cap and it is just a continuation of the feedline. However, the long cable length was tuned with a vector network analyzer for a 90deg delay at around 257.5MHz with the short and long cables terminated with 75ohms and the dipoles were then tuned for the best match with the harness inline.

I would make your test cables 1" longer and shorter to make any change more obvious.
prcguy

Right, I'll make something up tonight. Like I said, tip to tip, the long cable is within a millimetre or two of 13.5 inches. I can only assume that my tee-piece isn't quite right, or something else is throwing it out of whack.

I'll make a long cable of say, 13 inches, and one of 14 and see how it works.

It's a pain swapping cables though because my support tube is only 55mm in diameter (sorry, not sure what that is in inches) so getting fingers in there(!) to change things around is difficult.

If I have the theory right, the short cable is basically a continuation of the feeder, and the long cable is what sets up the delay - or is there some more matching going on in the 3.5 inch cable...? A fellow constructor on another forum made his antenna with the feeder going to one set of dipoles, and having a 10 inch length of RG-6 to the other set, and it works all right, from his tests anyway.

I understand why you used F connectors and tee-pieces - its easier (or in thereoy should be) for readers to reproduce the antenna if they're not messing about with soldering bare wires to each other, and I do take your point about the lack of braid on the RG-6.

Will report back when I've had chance to make up some alternate phasing harnesses.

David.

 

[Napalm]

I take it the length of wires from the F-sockets to the elements were taken into consideration too? (I think so, given what I've read on this thread).

The 'earthy' sides of my dipoles (elements C and A) are both 27cm long, from the tip of the elements to the nut on the F socket. The hot elements (B and D) are 28cm long (10 and a bit, and 11 inches, respectively).

Could this be causing some mismatch/problems? I can't really do anything to lengthen the earthy sides and vice versa.

Tried adjusting the phasing harness and I wasn't getting the results I expected, so I'm clutching at straws here

 

[prcguy]

The wires or lugs from the connectors to the elements are part of the elements and changing them from the example in the article may detune the antenna.

Some people have come up with great ideas using different diameter pipe and caps to hold everything together but you really need to make the business part of the antenna including the elements, coax harness and feedpoints exactly like the magazine article.
prcguy

I take it the length of wires from the F-sockets to the elements were taken into consideration too? (I think so, given what I've read on this thread).

The 'earthy' sides of my dipoles (elements C and A) are both 27cm long, from the tip of the elements to the nut on the F socket. The hot elements (B and D) are 28cm long (10 and a bit, and 11 inches, respectively).

Could this be causing some mismatch/problems? I can't really do anything to lengthen the earthy sides and vice versa.

Tried adjusting the phasing harness and I wasn't getting the results I expected, so I'm clutching at straws here

 

[Napalm]

The wires or lugs from the connectors to the elements are part of the elements and changing them from the example in the article may detune the antenna.

Some people have come up with great ideas using different diameter pipe and caps to hold everything together but you really need to make the business part of the antenna including the elements, coax harness and feedpoints exactly like the magazine article.
prcguy

Yeah 'guy, that's what I'm not understanding.

I bought the exact aluminium (sic) stock, the F connectors, the cable, the tee piece, the F sockets and even the wire to hook the dipoles up to the sockets, and its just not behaving right. Only thing I can think of to do now is take it all apart and re-build it, but frankly I don't have the drive to do that. I suppose the last remaining thing to do is to get the lugs you used, but the wires I'm using are the same length (they have to be, because my F sockets are in the exact place your PDF specifies. I marked it all out

My x-wing has potential, I just cannot get all the variables right.

 

[prcguy]

I'm wondering if your coax is the same construction and velocity factor as used on this side of the planet. Typical CATV or satellite RG-6 used here has foil shield with a thin braid and white foam type dialectic.

When cut to the specified lengths the long cable will be 13 1/2" tip to tip of the exposed center conductors and 13" face to face when measuring the inside bottom of the connector, which is the actual critical length. The short cable will be 3 1/2" tip to tip and 3" to inside connector bottoms.

I could probably send you a phasing harness if you think your coax is of different construction.

As a coincidence, today I gave away the prototype antenna pictured in the magazine to a good friend who collaborated on the project.
prcguy

Yeah 'guy, that's what I'm not understanding.

I bought the exact aluminium (sic) stock, the F connectors, the cable, the tee piece, the F sockets and even the wire to hook the dipoles up to the sockets, and its just not behaving right. Only thing I can think of to do now is take it all apart and re-build it, but frankly I don't have the drive to do that. I suppose the last remaining thing to do is to get the lugs you used, but the wires I'm using are the same length (they have to be, because my F sockets are in the exact place your PDF specifies. I marked it all out

My x-wing has potential, I just cannot get all the variables right.

 

[Napalm]

I'm wondering if your coax is the same construction and velocity factor as used on this side of the planet. Typical CATV or satellite RG-6 used here has foil shield with a thin braid and white foam type dialectic.

When cut to the specified lengths the long cable will be 13 1/2" tip to tip of the exposed center conductors and 13" face to face when measuring the inside bottom of the connector, which is the actual critical length. The short cable will be 3 1/2" tip to tip and 3" to inside connector bottoms.

I could probably send you a phasing harness if you think your coax is of different construction.

As a coincidence, today I gave away the prototype antenna pictured in the magazine to a good friend who collaborated on the project.
prcguy

I'd love to lie and say my cable is different, but its got a black plastic outer, thin copper braid, aluminium shield, solid white foam dielectric and a single copper inner conductor.

The reel it came off even says: RG-6, foam, black. Velocity factor 80%.

I'm starting to wonder if the environment I'm testing this antenna in is having an effect... but that could just be desperation I will give it one final try with it made to spec, later in the week, then I will try and get it working by a mixture of luck and 'suck-it-and-see' method.

I appreciate your patience!!!

ps: I lied, the cable is nothing like I said

 

[red-dog]

Per cavi che compongono la linea di adattamento, considerare la loro lunghezza tra "calza e calza" e lasciare il conduttore centrale il più corto possibile.

 

[red-dog]

For cables that make up the line of adaptation, considering their length between 'shoes and socks' and leave the center conductor as short as possible

 

[MarTay6]

Good Satellite Receiver??

I'm new on the forum here, found this thread very, very interesting. I'm interested in monitoring satellites- what kind of receiver recommendations can you make/what kind of handheld receivers to you utilize for this purpose??
Thanks-

 

[Napalm]

For cables that make up the line of adaptation, considering their length between 'shoes and socks' and leave the center conductor as short as possible

Let me try, instead of google, hi

For the phasing harness, the length needs to be calculated using the braid to braid distance and keep the centre conductor as short as possible (I think that bit was obvious).

I have given up with my X-wing for now - I cannot get it all to work right.

PRCguy: Were the element lengths calculated with the width taken into consideration? The wavelength is too short for 254MHz otherwise (I believe it puts it up at 270 or so)/

 

[prcguy]

The standard strippers for RG-6 leave 1/4" of center conductor exposed and 1/4" of braid exposed and that is how the article says to prep the cable. The 3 1/2" and 13 1/2" lengths of cable specified in the article would be tip to tip before or after stripping and when the connectors are installed properly with the cable dielectric flush with the bottom inside of the connector, the cables will be to spec, not including any difference in velocity factor.

The elements were tweaked to length using the specified material to provide the best match around 258MHz to the harness with the dipoles at the specified height.

If you assembled the antenna exactly as the article shows and its not performing it could be the harness and or connectors, Tee adapter or something is not the same as what I used. I will make and send you a harness and Tee adapter if you want, just send me a shipping address.
prcguy

Let me try, instead of google, hi

For the phasing harness, the length needs to be calculated using the braid to braid distance and keep the centre conductor as short as possible (I think that bit was obvious).

I have given up with my X-wing for now - I cannot get it all to work right.

PRCguy: Were the element lengths calculated with the width taken into consideration? The wavelength is too short for 254MHz otherwise (I believe it puts it up at 270 or so)/

 

[prcguy]

My receivers are a bit old but the best I've found so far is the Yupiteru MVT-7100. The older Radio Shack PRO-43 also works well.
prcguy

I'm new on the forum here, found this thread very, very interesting. I'm interested in monitoring satellites- what kind of receiver recommendations can you make/what kind of handheld receivers to you utilize for this purpose??
Thanks-

 

[MarTay6]

After further study, the Yupiteru indeed sounds like a great radio. A shame it's no longer available/here in the US. May still keep my eye peeled for a good used one. In the meantime, I've ordered an Alinco DJ-X11T receiver.... should do what I want to do at a price less than half that of the AOR receiver, as nice as it is.
Wes

 

[strandbiker1]

Tee adaptor

"A splitter will not work, it must be a Tee adapter. I've had good success with L-Com and here is a link to a page with the F Tee and male F to SO239"


Hello all! I put together one of the X wing antennas included the 4' X 4' reflectors. While my "F" T adapter looks like a T, versus a normal splitter (input on top and two outputs on the bottom, and typically 3 DB loss) it does indicate two outputs and one input on the face of it. Is this a true "T" adaptor, or is it actually a splitter in a physical "T" hardware package? I can hear the data channel on 250.55, but not very loud. I put together a double rhombic antenna and it works well!

Any ideas? I am right on the beach in Los Angeles. By the way, I hear the pirates (255.550) on the Rhombic at about 180 degrees south of me. The data channel (@50.550 is to the west of me by quiet a bit.

73's


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