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  • Author Author: kmikemoo
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Antenna Radiation - Phase 3

kmikemoo

Phase 3 of testing antenna radiation patterns.  The intro to this journey is Antenna Radiation.

Phase 3 will be to test a 2-element cardioid pattern.  This is formed when two 1/4 wave antennas are placed 1/4 wavelength apart and one is fed 90 degrees out of phase. I want to verify the single null to the rear.

 

The Amateur Extra amateur radio exam pool contains a question about this antenna pair and its unique radiation pattern - so, of course, I have to see if it's for real.  If it's what it's supposed to be, why isn't it common in Radio Direction Finding?

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Based on the description in the test question, I built a two element, 1/4 wave dipole pair.  Based upon the testing done in Antenna Radiation - Phase 1, I do know that the antenna feed line will influence the radiation pattern.

I was able to use the automated transmitter (fox) and record the readings with my cellphone.  Three 30 second segments for each of the eight directions (0, 45, 90, 135, 180, 225, 270, 315) and an additional 30 seconds back at 0/360 - just to be sure.

I did not get the pattern that I expected.  The graph is a top down view.

imageimage

The in-phase feed point is at the center, both of the dipole mounted in line with the vertical support and of the graph.  The element to the right in the picture is 1/4 wavelength (+/-) away, resulting in a 90 degree phase shift of the transmitted signal at (again) 1/4 wavelength spacing.  This is what creates the large lobe to the right on the graph as 0 / 360 is the direction of the second element.

This is an awesome directional pattern and the nulls were clean (zero) and very distinct.  Too bad this wasn't the pattern I was looking for.  So... where did I go wrong?

 

DI-pole.  Two.  While the elements are a quarter wavelength long, there are two.  This makes my dipole a half wavelength antenna.  My error was assuming that the lower elements would act as a ground plane - which they did not.  I will have to add more counterpoise elements to try to create a ground plane.  Maybe I can put something together with whip antennas...

I'm starting to see why this antenna array might not be the system of choice for radio direction finding.

 

After thinking about it, I decided to turn the dipole pair into two 1/4 wavelength ground plane antennas.  Surely, that will give me the pattern that the Amateur Radio exam question references.

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Yeah... no.  Wrong again.  I got the same radiation pattern with the ground plane antennas as I did with the dipoles.  The magnitude of the readings did not change if the lower dipole elements were connected to the ground plane elements or not.

 

Given that this pattern is accepted as a universal constant in antenna propagation, I must be doing something wrong.  It must be that the phase shift is more than feeding the radiator element 90 degrees later.  If the arrangement I have is feeding the radiating elements in phase, I should have symmetrical lobes - at least according to my antenna books and all of the web sites that I've visited in the last few weeks.  I do not have symmetrical lobes.  Time to restore the antenna back to the twin dipole configuration and see about creating a 90 degree shift in the feed lines to the forward radiating element.

 

After some Internet research, it doesn't look like I'll be able to create that 90 degree phase shift without spending a bunch of money - and nothing would arrive in time anyway.  Argh!

 

So I shifted to "What CAN I do?"  Well... I can try simple.  A capacitor.  I only have one that can handle the voltages.  It does nothing.  Not surprised.

 

What if I shift the feed line of the second radiating element?  Sure, if I connect the top radiating element at the first feed point to the lower radiating element on the second feed point it is a 180 degree shift, but I should see SOME change in radiation pattern.  The pattern is a decent approximation.  I did three laps around the antenna confirming the peaks and valleys and rough readings.  At least the pattern changed.

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Again, the main vertical support is also the center of the graph.  The second dipole is to the right along the zero axis.  The measurements are made at two meters from the vertical support.  The measured values are doubled to illustrate the pattern.  At least I know that the radiation pattern can be changed.

 

One of my Yagi antennas has a 1/2 wavelength balun.  What if I try something like that?  I returned the dual dipoles to their original configuration and connected a 1/4 wavelength piece of coax to the second radiating dipole such the the center conductor (connected to both upper radiating elements) terminated on the feed point to the lower element of the second dipole.  The coax shield went from the lower element on the second dipole to the upper element.  Think: Feed point, feed point, shorting loop.

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I would say that the 1/4 wavelength "balun" effectively eliminated the second dipole because the resulting pattern looks a lot like the original single dipole from Phase 1.  Again, the measured values were doubled for illustrative purposes.  Neither variant was pegging the meter.

 

Unless I stumble across a way to give me a 90 degree phase shift in the next week, I will not be able to see the cardioid pattern in real life.  This exercise does demonstrate why this antenna setup is not used more often in Radio Direction Finding.  Time Difference Of Arrival is a simple enough circuit and uses the venerable 555 timer.  I was not able to find something quite so elegant for the 90 degree phase shift.  I'm not done looking... just bitter. Ha! Ha!

 

This was an awesome experience and I thank Element14 for selecting RF.  I would not have been motivated enough without the challenge.  I appreciate the opportunity to actually see phenomenon that I've only read about.  I will definitely play with this more once the weather improves.  I originally planned to scrap the dual dipoles after this, but after seeing how clean their radiating pattern is... I'm going to find a use for them.

73, Mike, WI9MMM

 

UPDATE:  8 JAN 2020

Huge thanks to Jon Clift (  ) and Shabaz (  ) for their wisdom.  I ran some simulations using EZNEC, the free version of an antenna modelling program.  Given some allowance for the feed line within the radiation pattern, I would say that this is similar to what I measured.

   imageimage

The speed of the signal through the coax - despite the short physical length - caused my second dipole to radiate at about 135 degrees out of phase.  I am using RG8x coax with a velocity factor of about 0.8.  If I go for a 270 degree shift and account for the velocity factor, I should still get the cardioid pattern - only on the opposite side of the pair.  I am curious to see how much the radiation pattern is affected by the extra feed line within the field.

 

Running a few more simulations, I discovered that a 270 degree shift would result in high SWR but a 450 degree shift (1-1/4 wavelengths) should be fine.  BUT... the velocity factor range of my RG8x is 0.79 to 0.84 - according to the internet.  Being off by a few inches can skew the pattern.  Window line, similar to the antenna wire on our old televisions, has a velocity factor of 0.92 to 0.95 and just wire is supposed to have a velocity factor of 0.95.  0.05 difference in the RG8x.  0.05 difference of wire to open air.  I'll try just wire.

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Yes, that is just two wires connecting the dipoles.  It would be the same thing as window line - only with a bit more spacing.  I did need to change out the radiating elements for longer elements to get the antenna's SWR to be its lowest in the 144MHz to 148MHz range.  I fully expected the unshielded connector to severely mess with the radiation pattern would it be that much more than the coax?  We'll find out.

image

WOW!  Is that an ugly pattern, or what?!  BUT... the null is definitely there and it is where it should be.  I have not figured out how to model how the feed line affects the pattern but from previous experiments, I know that it decreases the radiated signal strength around it.  On this experiment, the feed line only extended away from the feed point.  I needed to use it to "anchor" the antenna to keep it from thrashing about in the wind.  I also added some simple reference points.  The resulting null is fairly shallow due to the feed point phase difference being past 90 degrees (I estimate around 105 degrees) and, more significantly, RF being radiated from the feed line and the 1/4 wave connector.  I'm confident that I could get a deeper null if I shifted to coax, but not sufficient enough a null to be discernible for Radio Direction Finding.

 

Mission complete.  Return to base.  Clean up shop. image

Parents

  • Good experiments. This is fun reading these blogs.

     

    So... where did I go wrong?

     

    From what little I know of rf stuff, my guess is it relates to the coax feed. Your elements are physically 1/4 wavelength apart, but the wave velocity along the coax is only 2/3rds the speed of light (roughly 2/3rds, it will depend on the actual cable you've used), so you're not driving the front pair 90 degrees out of phase with the back pair (it's more like 60 degrees). See if it comes right if you simply extend the coax length by roughly half as much again [multiply by the inverse of the velocity factor for the actual cable if you have the details for it].

     

    By the way, paralleling the dipoles like this isn't necessarily a very nice load for your transmitter. If it's a very valuable piece of kit, you may want to reconsider what you're doing.

     

    The other thing that stands out from your pictures is how you bring the feed down near the lower dipole element. I would think you would need to move the feed further back if you want really clean radiation patterns. The lower dipole element isn't a ground - it has rf currents running in it and radiates [think of it as being driven by the top element through the electric field that forms between the two]. Having the shield of the coax nearby has as a couple of effects. It will mess up the symmetry of the electrical field between the higher and lower elements of the dipole, so skewing the radiation pattern. Also, to some extent it will passively receive and reradiate. Both will mess up your radiation patterns a bit (and in quite a complex way where you've got it coming across on the diagonal).

  • in reply to jc2048

    I'm wrong, I've got it back to front. [I did say I didn't know a lot about RF!]

     

    At 144Mhz, a quarter wave in air is 0.52 metres.
    A quarter wave in coax with a VF of 0.66 is only 0.3425 metres.

     

    So as you've got it, with a simple direct run of coax, the signal can't get there quickly enough.

     

    So, what other options might you have?

     

    Making the coax one-and-a-quarter wavelengths should work, that would give you a piece of coax that would stretch between the dipoles, but gives you quite a long length of coax [1.7125m] to coil up.

    Another possibility would be three-quarters [1.0275m], which might be a bit more manageable, with the front dipole inverted [180 degrees, so effectively and an extra half wavelength].

     

    Or you could have two feeds up to the antenna and adjust the lengths further back where it is all out of the way.

Comment
  • in reply to jc2048

    I'm wrong, I've got it back to front. [I did say I didn't know a lot about RF!]

     

    At 144Mhz, a quarter wave in air is 0.52 metres.
    A quarter wave in coax with a VF of 0.66 is only 0.3425 metres.

     

    So as you've got it, with a simple direct run of coax, the signal can't get there quickly enough.

     

    So, what other options might you have?

     

    Making the coax one-and-a-quarter wavelengths should work, that would give you a piece of coax that would stretch between the dipoles, but gives you quite a long length of coax [1.7125m] to coil up.

    Another possibility would be three-quarters [1.0275m], which might be a bit more manageable, with the front dipole inverted [180 degrees, so effectively and an extra half wavelength].

     

    Or you could have two feeds up to the antenna and adjust the lengths further back where it is all out of the way.

Children
  • in reply to jc2048

    I appreciate the feedback.  This kind of discussion was what I was hoping to get from this.

     

    I did consider a 3/4 wavelength connection between the two dipoles so that I could account for the RG8x velocity factor.  I figured that with the approximately 1/4 wavelength connection, knowing that it wouldn't be a perfect 90 degrees electrically, I should get me a pattern somewhat like what I was expecting.  Not even close.  When I got the pattern that I did, I knew that some of my assumptions were wrong.  I would like to understand why I got what I did.

     

    The coax does affect the radiation pattern.  The method of running the feed line perpendicularly away from the feed point is based on both experimentation and the recommendations of seasoned amateur radio folks (Elmers).  I needed to do this to decrease my SWR (Standing Wave Ratio) - basically, reflected power as I understand it.  The impact is shown on the radiation pattern of the single dipole.  The effect of the coax cut-back to the main support was not tested at all on the dual dipoles.  The cutback attaching to the vertical support at approximately the same level as the bottom of the bottom radiating element yields the best SWR.  I do also know that allowing the coax feed line to hang near the lower radiating element increases the SWR.

     

    I never thought about the radiation pattern of the connecting coax.

     

    I did also consider two feeds.  Because the measured pattern is that far off of target, I didn't pursue.  I'll admit that the snow and freezing rain may have influenced my decision.

     

    For the record, I do check and adjust the SWR of the antennas.  So far, the highest SWR I've been willing to test with is 1.4.  I've read that as long as I keep it below 1.5, I shouldn't have to worry.  It's also why I'm using a $35 radio as my transmitter.

  • in reply to kmikemoo

    I'm not an RF engineer, so if your Radio Amateur friends say one thing and I say another, go with what they say (they're the ones with real experience). I can understand some of this, partly from general principles and theory, but also from working with EMC (where you realise, after you've been doing it for a while, that you're reversing into rf engineering and gradually understanding what were once regarded as the 'black arts').

     

    An important thing to remember with high frequencies is that currents flow close to the surface [skin effect]. So the return current, for the signal that goes down the centre conductor of the coax, flows near the surface of the inside of the shield. It's on the inside because it wants to be as close as possible to the signal it's associated with. Although it seems counter-intuitive to what we're used to at DC, where a current can only flow in one direction down a conductor, in this case a second rf current can flow on the outer surface of the screen without interacting with the one inside. It can even flow in the opposite direction to the one inside. Essentially, the outside can then behave just as a passive director or reflector would, receiving a signal and then re-radiating it. [Real life is, of course, more messy and there will often be some small currents on the outside of the coax. In situations where people are doing critical measurements, like EMC testing, you'll often see them clip ferrites to both ends of the antenna connecting cable to absorb any outer-surface currents.]

     

    What your friends are saying about the diagonal is that if you have the choice between parallel to the dipole or on the slant, go for the slant. That's sensible if your feed has to be close to the dipole as it disrupts any director/reflector effect as much as possible. But, that said, it still should be better to get it as far back as possible [because the further away, the less the signal received and reradiated, and the less the effect it then has when it gets back to the dipole].

     

    This is how I'd explain the cardioid pattern.

     

    image

     

    When the wave radiated by dipole A arrives at B (a quarterwave away), it's 90 degrees on. If B is fed with a signal that's also 90 degrees on, then the two add in the forward direction (constructive interference). However, the signal radiated by B in the reverse direction, when it gets to A, will be 180 degrees on from the signal emitted by A in the reverse direction and the two will subtract (destructive interference, if you want to think of it in wave terms). So that's why the strong null in the reverse direction. But it only applies to the reverse direction along the axis joining the dipoles, as soon as you move away from that axis the cancellation starts to fall and the signals start to progressively add more and more as you come round.

     

    To get a good null, everything will need to be right. The distance betweeen the dipoles, the 90 degree shift by delay in the coax for the second dipole [which will be the hardest to get right], and the alignment (the dipoles need to be accurately in the same plane).