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?
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.
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.
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.
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.
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 (jc2048) and Shabaz (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.
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.
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.
WOW! It 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.