Co-phasing LPDA questions

[zguy1243]

I am in the middle of a project where I am "stacking" or co phasing two 225-400Mhz LPDA's. My antennas will be vertically polarized. My question is, should I stack the antennas horizontally or vertically on the support structure? I understand that stacking the two antennas will narrow the beam width thus producing a little more gain, close to 3db I hope. I understand that if you stack vertically polarized antennas vertically vs. horizontally on the mounting structure it will produce gain but in a different manner. It will narrow either the vertical or horizontal beam width of the antennas. My question is this, should I mount them vertically stacked or horizontally stacked considering them being vertically polarized antennas. My spacing of the antennas will be 1/2 wavelength of the lowest operating frequency (225Mhz) which is 26.24 inches. My connecting harness will be two equal length 50 ohm jumpers to a mini circuits power divider/combiner and on from there to mast mounted 400Mhz low pass filter and Angle linear preamplifier.

Thanks in advance for any ideas on this....

 

[ka3jjz]

Can you even guarantee the performance of a phasing harness across such a wide range of frequencies? I have my doubts. I suspect as the operating frequency gets further and further away from the design, the harness is actually going to cause more problems than it solves.

??? 73 Mike

 

[zguy1243]

Can you even guarantee the performance of a phasing harness across such a wide range of frequencies? I have my doubts. I suspect as the operating frequency gets further and further away from the design, the harness is actually going to cause more problems than it solves.

??? 73 Mike

Well I am shooting for the 225-400 mhz band, 175 Mhz wide. To my knowledge as long as the I am using a coupler/divider with the correct freq range I should be good for the 175Mhz using balanced line from both antennas of equal length. With a true phasing harness you are good for a small range because you are calculating multiples of the wavelength at one frequency. The way that I am using a power divider/combiner I will be combining both signals with identical signals so I should be ok across the band of 175 Mhz I think, or this is how I understand it. This combiner that I have is for 225-400Mhz and the spec sheet shows to pass or split/combine equally from the two feed ports to the main feedline output. So if I have exact antennas, exact same length jumpers from said antennas, both antennas feeding this combiner/divider with equal voltages at this point I should be ok, again, I think...

 

[jackj]

I think that I would stack them horizontally, they will require less space and your phasing harness can be shorter. Your 3 db of gain comes from the fact that you have two signal sources feeding the same load in phase with each other. That fact will double the received power which is a 3 db increase. I think you will find that your beam width will not change much, if at all.

Edit:

I completely missed the fact that you are going to use a combiner. The loss in the combiner will offset any gain you might have from using 2 antennas. Sorry.

 

[prcguy]

I'll check with my antenna mentor but I think you are taking the only viable approach. Since you are using identical antennas and combining with a power divider the length of the two 50ohm feed lines after the power divider are not frequency dependent and can be any length as long as they are the same length.

I believe the correct way to space log antennas for vertical pol is side by side and place the high frequency ends at the recommended spacing for best gain and do the same for the low frequency ends of the antennas. This will place them looking "cross eyed" towards each other in the direction of the high frequency ends.

In the end it would probably be a lot cheaper to make a single log periodic antenna with a boom twice as long with twice as may elements giving you about the same improvement as stacking two separate antennas.
prcguy

 

[prcguy]

Gain and beamwidth go hand in hand. To get more gain you get narrower beamwidth somewhere.
prcguy

I think that I would stack them horizontally, they will require less space and your phasing harness can be shorter. Your 3 db of gain comes from the fact that you have two signal sources feeding the same load in phase with each other. That fact will double the received power which is a 3 db increase. I think you will find that your beam width will not change much, if at all.

Edit:

I completely missed the fact that you are going to use a combiner. The loss in the combiner will offset any gain you might have from using 2 antennas. Sorry.

 

[kb2vxa]

You think but you think wrong for the most part. Think PHASING, stacking (paralleling) produces the correct phase angle(s) only across a very narrow range of frequencies, roughly 2MHz or less. Everything else is hit or miss, mostly miss. When it comes to beam characteristics we're talking a whole 'nuther science here, one WAY beyond the scope of an internet forum.

 

[zguy1243]

I think that I would stack them horizontally, they will require less space and your phasing harness can be shorter. Your 3 db of gain comes from the fact that you have two signal sources feeding the same load in phase with each other. That fact will double the received power which is a 3 db increase. I think you will find that your beam width will not change much, if at all.

Edit:

I completely missed the fact that you are going to use a combiner. The loss in the combiner will offset any gain you might have from using 2 antennas. Sorry.

I have seen many HAM setups that use just this combiner type approach for 2 same antennas that would exhibit the same supposed 3 db loss. If there is not some kind of gain to be had other than changes in beam width in one plane or another why would it be employed?

 

[prcguy]

Think two identical 50ohm wide band log periodic antennas combined with a 50ohm power divider rated for the same frequency range and the specific frequency elements of each antenna are spaced at the correct distance apart across the entire frequency range of the antenna. This means at any specific frequency the spacing would be the same as stacking two identical Yagi's at a single frequency.
prcguy

You think but you think wrong for the most part. Think PHASING, stacking (paralleling) produces the correct phase angle(s) only across a very narrow range of frequencies, roughly 2MHz or less. Everything else is hit or miss, mostly miss. When it comes to beam characteristics we're talking a whole 'nuther science here, one WAY beyond the scope of an internet forum.

 

[jim202]

Let me throw one more point to ponder here. Most 2 way splitters or power dividers have a 3 db insertion
loss. Generally when you combine antennas of the same number of elements you get a 3 db gain. It
would seem that there is a zero balance here between the power splitter and the added 3 db gain of the
2 antennas. If that is the case, why are you even trying to do this?

Jim

Think two identical 50ohm wide band log periodic antennas combined with a 50ohm power divider rated for the same frequency range and the specific frequency elements of each antenna are spaced at the correct distance apart across the entire frequency range of the antenna. This means at any specific frequency the spacing would be the same as stacking two identical Yagi's at a single frequency.
prcguy

 

[zguy1243]

Let me throw one more point to ponder here. Most 2 way splitters or power dividers have a 3 db insertion
loss. Generally when you combine antennas of the same number of elements you get a 3 db gain. It
would seem that there is a zero balance here between the power splitter and the added 3 db gain of the
2 antennas. If that is the case, why are you even trying to do this?

Jim

If I find any gain at all with phasing two of these antennas it would be great and this would push me to stack four of them to achieve some higher gain numbers. My Mini Circuits power divider/combiner has a insertion loss of 1.68 Db, so I am thinking that I should see a little gain. My gig has been DX'ing aircraft signals for a good while. I have done many upgrades to my receive gear, some of them were only fractions of a db gains but I have done this kind of thing many many times and all of those tiny gains have added up. Right now there are a few unknowns for me in regard to stacking these antennas. I am not sure about the spacing of the antennas. My motivation to try this came from a run in with some stacked TV antennas that I ran across at a customers house (a TV DX'er of sorts). I was very impressed with what he had done with some stacked TV log periodics. Back to what I was saying, if there is any gain at all it would be worth it to me. I don't have much room for further improvement anywhere else in my system, I am using hardline cable, bandpass filters, high end low noise preamps, custom built antennas, pretty much the best I can get at every corner I have turned.

 

[benbenrf]

Zguy1234

Your receive beamwidth will be dependent on the E and H plane beam-width(s) of the LPDA as a single anttena i.e. if your LPDA's have an H-plane beam-width that is wider than than the E-plane, vertical stacking may not necessarily give you a narrow beam-width.

However, as a rule; the -3dB H-plane beam-width of an LPDA tends to be wider than the -3dB E-plane beamwidth.

There are lots of issues to take into consideration when deciding which stacking arrangement to go for.

The E and H plane of the antenna (as a stand alone antenna) should not be ignored – especially at VFH/UHF freq's with broad band LPDA's – a study of the LPDA radiation patterns will almost certainly show a considerable variation and change in lobe shape & size along the horizon as frequency changes. LPDA's tend to demonstrate increased lobe fracture and breakup as frequency rises within their design bandwidth. However, what is certainly true is that vertical stacking tends to offer a better S/N ratio, a reduction in received ground noise and multi-path reflections, and in that respect it is better than Horizontal stacking.

Both vertical and horizontal stacking can suffer form angle of arrival problems. In verticly stacked antenna's, any signal above or below the centre line stacking distance is in theory going to be out of phase with respect to one or other antenna (from below it will be out of phase with respect to the higher mounted LPDA, and from above it will be out of phase with respect to the lower mounted antenna). How many degrees out will be a function of the angle of arrival and vertical mounting distance between the 2 antenna's. The exact same rule applies to horizontally mounted antennas, except now the phase error in degrees is a function of horizontal distance between the antenna's and horizontal angle of arrival off the centre line between the 2 LPDA – and yes, a vertical off-centre angle of arrival is an addition signal compenent that can effect phasing error.

… and we can go on and on. In reality, the nature of VHF and UFH listening, the quality of equipment in use amongst consumers, the dBm level of most transmissions and the construction error in nearly all LPDA's means that things like phase error, received ground noise, thermal factor, blah blah, blah blah …..... are going to have little real world impact on "listening experience". Now, if your usual received signal is a UWB signal, or high-speed multiplexed CDMA type signal – then yes, things like phase/multi-path error stemming from vertical/horizontal mounting error, can quickly add to system noise to stuff things up .... but for day to day VHF/UHF listening – nope, you are going to experience little real world impact to the quality of the received signal. On the other hand zguy1234, a quick look at your Rx gear gives me the impression that you sure like to use decent gear, and you therefore sur elike to have things setup properly.

Single errors in hardware setup seldom effect received signal quality, the problem is: single errors can quickly add up to become collective errors, and collectively all the little errors in setup do degrade received signal quality.

There are arguments for and against vertical versus horizontal stacking. If you follow through with the following, you will have deal't with 90% plus of the real world potential problems you could suffer with stacking:

- Power split: a divider (located equi-distant from the driven element of both LPDA's).

- phasing: phased coax between the divider and the driven element (i.e. same type coax with same type connectors), and, of course, both pieces of equal length.

- stacking rule: there is no optimal stacking distance for LPDA's – ideal stacking distance is a function of antenna resonant frequency, and LPDA's are broadband so the ideal stacking distance is only every going to occur at one frequency! Space your LPDA booms ½ the distance of the antenna's vertical aperture and all is going to be fine.

..... and last but not least: the biggest impact on performance by far, other than for coax quality between antenna & receiver, is going to come from LPDA raised height and correct antenna/boom/mast earthing - get them up as high as possible and get them well earthed and you will have accounted for 95% plus of the issues on your side that stand to effect received signal quality.

Good luck

 

[zguy1243]

Great Information. I have read over this a few good times and done some serious reading on some of the key points that you have touched on and I am gaining some good understanding now. Right now I am working with a local machine shop doing some fabricating of the support mast. I am also waiting on a 18 foot Glen Martin roof tower that I will be mounting this thing out of. I will update this thread with photos and of course some youtube videos of the results. Thanks to all that have responded and that continue to comment.

Zguy1234

Your receive beamwidth will be dependent on the E and H plane beam-width(s) of the LPDA as a single anttena i.e. if your LPDA's have an H-plane beam-width that is wider than than the E-plane, vertical stacking may not necessarily give you a narrow beam-width.

However, as a rule; the -3dB H-plane beam-width of an LPDA tends to be wider than the -3dB E-plane beamwidth.

There are lots of issues to take into consideration when deciding which stacking arrangement to go for.

The E and H plane of the antenna (as a stand alone antenna) should not be ignored – especially at VFH/UHF freq's with broad band LPDA's – a study of the LPDA radiation patterns will almost certainly show a considerable variation and change in lobe shape & size along the horizon as frequency changes. LPDA's tend to demonstrate increased lobe fracture and breakup as frequency rises within their design bandwidth. However, what is certainly true is that vertical stacking tends to offer a better S/N ratio, a reduction in received ground noise and multi-path reflections, and in that respect it is better than Horizontal stacking.

Both vertical and horizontal stacking can suffer form angle of arrival problems. In verticly stacked antenna's, any signal above or below the centre line stacking distance is in theory going to be out of phase with respect to one or other antenna (from below it will be out of phase with respect to the higher mounted LPDA, and from above it will be out of phase with respect to the lower mounted antenna). How many degrees out will be a function of the angle of arrival and vertical mounting distance between the 2 antenna's. The exact same rule applies to horizontally mounted antennas, except now the phase error in degrees is a function of horizontal distance between the antenna's and horizontal angle of arrival off the centre line between the 2 LPDA – and yes, a vertical off-centre angle of arrival is an addition signal compenent that can effect phasing error.

… and we can go on and on. In reality, the nature of VHF and UFH listening, the quality of equipment in use amongst consumers, the dBm level of most transmissions and the construction error in nearly all LPDA's means that things like phase error, received ground noise, thermal factor, blah blah, blah blah …..... are going to have little real world impact on "listening experience". Now, if your usual received signal is a UWB signal, or high-speed multiplexed CDMA type signal – then yes, things like phase/multi-path error stemming from vertical/horizontal mounting error, can quickly add to system noise to stuff things up .... but for day to day VHF/UHF listening – nope, you are going to experience little real world impact to the quality of the received signal. On the other hand zguy1234, a quick look at your Rx gear gives me the impression that you sure like to use decent gear, and you therefore sur elike to have things setup properly.

Single errors in hardware setup seldom effect received signal quality, the problem is: single errors can quickly add up to become collective errors, and collectively all the little errors in setup do degrade received signal quality.

There are arguments for and against vertical versus horizontal stacking. If you follow through with the following, you will have deal't with 90% plus of the real world potential problems you could suffer with stacking:

- Power split: a divider (located equi-distant from the driven element of both LPDA's).

- phasing: phased coax between the divider and the driven element (i.e. same type coax with same type connectors), and, of course, both pieces of equal length.

- stacking rule: there is no optimal stacking distance for LPDA's – ideal stacking distance is a function of antenna resonant frequency, and LPDA's are broadband so the ideal stacking distance is only every going to occur at one frequency! Space your LPDA booms ½ the distance of the antenna's vertical aperture and all is going to be fine.

..... and last but not least: the biggest impact on performance by far, other than for coax quality between antenna & receiver, is going to come from LPDA raised height and correct antenna/boom/mast earthing - get them up as high as possible and get them well earthed and you will have accounted for 95% plus of the issues on your side that stand to effect received signal quality.

Good luck

 

[Mork]

Is a couple of db really worth it?

Interesting theoretical discussion, however, are you really going to hear the possible 3db gain in terms of s/n ratio on Mil-Air AM? Is it really going to be worth the time and effort? I'd shoot for increasing the antenna height and adding a quality low noise Mil-Spec pre-amp at the antenna. Granted an excellent GaAsFET pre-amp is not going to present the bandwidth but look at the possible gain. More bang for the buck IMHO.

p military

doc

 

[zguy1243]

Yeah I believe that it will be worth it. I have done about everything else possible in regard to preamps (I use the same ones as NASA uses in California to talk to the shuttle, cable, tower, radios, connectors , filters, to the tenth power. Besides its fun, gives me a excuse to get into something

Interesting theoretical discussion, however, are you really going to hear the possible 3db gain in terms of s/n ratio on Mil-Air AM? Is it really going to be worth the time and effort? I'd shoot for increasing the antenna height and adding a quality low noise Mil-Spec pre-amp at the antenna. Granted an excellent GaAsFET pre-amp is not going to present the bandwidth but look at the possible gain. More bang for the buck IMHO.

p military

doc

 

[benbenrf]

zguy1234

Is a true 3dBi gain going to be realised? ...........

Yes & and No - depends where/how and just what "gain" is been referred to.

If the improvements are to be considered in terms of traditional "antenna gain", I'd be surprised if much more than around 2,2dBi (at most) in real terms was realized ....... still, not a figure to be sneezed at, granted, and by its self a perceivable improvement to demodulated audio/video/data quality (i.e. you'll be able to see/hear the improvement).

But to plan changes to an antenna setup against a theoretical increase in gain (worked out and calculated on a piece of paper?), is quite often to miss out on what can often be far more important & relevant.

The increase in received signal strength that comes through antenna "doubling-up" is almost always beneficial & welcome (and I say "almost always", because it is not always a good thing - there are occassions where "doubling up" can be a bad thing), the most significant "gain" in this project I believe is going to come about through improvement to the S/N ratio.

On top of the antenna "doubling-up gain" (which, as said, I suspect is going to be in real world terms no more than around 2,2dB at most) there is going to be the tighter & better defined E/H plane's of the stacked array - the listening aperture of this array is going to improve significantly and lead to improved demodulated signal audio/video/data - more so than the +/-2db change in received signal strength.

In short: signal CLEANLINESS and QUIETNESS is going to be greater the greater benefit here than any gain in realized signal STRENGTH.

The classic representation of just how much difference a better S/N ratio makes, is as most will know, experienced with HF loops. Loops offer no gain improvement over wire type antenna's - their gain is often, when replacing wire type antenna's, -10, -20, -30, -40 and even as much as -50 or -60dB worse, but, the reduction in noise pickup they offer is soooooooo.... much that gain reduction becomes a non-issue. The demodulated signal is cleaner. quieter, clearer and easier to listen to. Yes, noise phenomena is different on HF bands, but the principal none-the-less remains the same across VHF/UHF bands.

Experience leads me to suspect zguy1234's big improvement/benefit is going to come from the improved S/N ratio he will experience, more so than from the increase in received signal strength that is going to be presented to the receiver input ...... which leads onto "pre-amps".

What about adding a preamp?
Generally I am hesitant when it comes to pre-amps. As you will no doubt know, pre-amps not only amplify the all important modulated content of a signal, they also amplify all the noise.

If there is no question that failure to demodulate is because of signal weakness, as opposed to noise content, then yes, a pre-amp is the way to go. But otherwise - nope, a pre-amp will serve no benefit.

In this case, the big improvement to S/N ratio that is going to come about from changes in antenna array setup, offers a brilliant opportunity to exploit the benefits of a preamp - take a look at Kuhne electronic - Low Noise Amplifiers - these are brilliant pre-amp products with some of the broadband ones offering very low noise figures (though, like antenna gain, the noise figure in pre-amps is not by its-self always a reliable indicator of pre-amp performance).

Yes ..... please post up some pic's - I'd be keen to see some of theLPDA youre using - check out the pic of the WJ - LPDA I posted on a recent thread by MadSpleen85 “Antenna in the Attic and Coax question” (I have a whole bunch of them in the attic if you're interested).

Let me end off by adding/asking: horizontal LPDA array's generally allow for easier azimuth/elevation changes, than do vertical setups (well, it's generally less mechanics) - how are you intending to carry out elevation changes (which you're going to need to do if you want to exploit all the sat signals)?

 

[zguy1243]

Here some photos of the antennas that I am using. This antenna I had custom built for the 225-400Mhz spectrum. It flat smashes anything I have used for 225-400Mhz. Its a well built, well designed log. I made a point in incorporate heavy duty rear mounting to avoid mast interaction. I plan on only using this antenna for DX aircraft comms in the 225-400 spectrum. I have some nice mil surplus satcom antennas that do great so I will not be needing to change elevation just azimuth, I have a nice CDE Ham IV rotor for this. I showed a pic of the pre amps that I am using. The Angle linear amp is a PHEMPT (Psuedomorphic High Electron Mobility Transistor) amp, the noise figure is less than 1/2 of a DB. I had this amp special built and optimized for the 225-400Mhz band by Chip Angle at Angle Linear. Check out his site at Angle Linear Home Page. I agree with you totally about the gain seen from reducing the S/N ratio. the example with HF sums it up very well. Bandpass filtering is another aspect that should not be overlooked. I see some of the best improvements in S/N by adding high or low pass filters. I would be curious about the watkins antenna. Whats the freq range of the antenna?

 

[zz0468]

My Mini Circuits power divider/combiner has a insertion loss of 1.68 Db, so I am thinking that I should see a little gain.

What splitter/combiner has a loss of only 1.68 db? Mini-Circuits sometimes specifies loss as "loss above theoretical", so a 1.68 db spec really means 1.68 db ABOVE 3 db.

Example: http://www.minicircuits.com/pdfs/ZAPDJ-2.pdf

Your splitter likely has a loss of 4.68 db total.

The loss of a practical, well-made splitter/combiner will be in excess of 3 db, therefor the theoretical gain of your stacked antennas will be less than with a single antenna.

If you were to phase the antennas with phased cables, no combiner, you could see close to the theoretical 3 db maximum gain (minus cable losses), but only on one specific narrow range of frequencies.

Bottom line: Your idea simply will not work without violating the laws of physics.

Edit: Many of us have seen phased LPDA's, particularly at cable tv head end sites. The caveat there is, they're generally being used on a much narrower range of frequencies, and are 'cross-eyed' in the manner that prcguy stated above. They are also NOT using a splitter/combiner

 

[zguy1243]

So far I have gotten a split response on the idea. It all makes sense but I am the type that has to see for myself. I am going to throw this stuff together and see what gives. I appreciate all of the informative posts. If all I end up with in the end is some knowledge, well that would be considered a gain to me just as well.

What splitter/combiner has a loss of only 1.68 db? Mini-Circuits sometimes specifies loss as "loss above theoretical", so a 1.68 db spec really means 1.68 db ABOVE 3 db.

Example: http://www.minicircuits.com/pdfs/ZAPDJ-2.pdf

Your splitter likely has a loss of 4.68 db total.

The loss of a practical, well-made splitter/combiner will be in excess of 3 db, therefor the theoretical gain of your stacked antennas will be less than with a single antenna.

If you were to phase the antennas with phased cables, no combiner, you could see close to the theoretical 3 db maximum gain (minus cable losses), but only on one specific narrow range of frequencies.

Bottom line: Your idea simply will not work without violating the laws of physics.

Edit: Many of us have seen phased LPDA's, particularly at cable tv head end sites. The caveat there is, they're generally being used on a much narrower range of frequencies, and are 'cross-eyed' in the manner that prcguy stated above. They are also NOT using a splitter/combiner

 

[zz0468]

So far I have gotten a split response on the idea. It all makes sense but I am the type that has to see for myself. I am going to throw this stuff together and see what gives. I appreciate all of the informative posts. If all I end up with in the end is some knowledge, well that would be considered a gain to me just as well.

Well ok... Just realize that you're inserting more known, specified loss into your system than theoretically possible gain, but if you're happy with the result, that's all that matters.