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Antenna Interaction?




 
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Todd, KA1KAQ
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« on: January 13, 2009, 01:41:41 PM »

Still in the planning stages here for the radio set up and probably making more out of this than really exists. But before getting folks here to help string the dipoles, it'd be good to know if I need to rethink the layout.

Trees, especially tall trees, are few and far between on our property. Fortunately there is one cluster of fairly tall pines (50-60 feet) on the northeast side and two tall hemlocks (60+ feet) on the west side, perfect for stringing at least the 160 half wave between. Further down the yard to the south is a row of maples in the 30-40 ft range, and some pecan trees probably 40 ft or so, running east to west across the front yard.

Both aerials will run East-West almost exactly. The 160 dipole should fit fine between the pines and the back northerly-most hemlock, probably around 60' high. The 80m dipole at this point looks like it will fit best between a pecan tree and hemlock #2, probably between 40-50' high at most. The problem is, hemlock #2 is in line with #1, maybe 25-30' south of it. The wires won't look directly at each other in the same plane, but will be in fairly close proximity RF-wise.

I'm not concerned about any impact on my 160 dipole looking south. But looking north, the 80m will be firing right at/through the 160m to some extent. It's bound to screw with the pattern, but I'm more concerned with making some odd-ass harmonics or other garbage that wipes out every TV and Wii in the neighborhood. Or will the 160m act more like a reflector to my 80m signal? Or (most likely) am I making something out of nothing with all of this?

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« Reply #1 on: January 13, 2009, 02:22:55 PM »

Todd,

I think your only concern would be leaving a receiver connected to one antenna while transmitting on the other, it might not make the receiver input very happy.  Otherwise, I would not be concerned about any interaction effect.

Typical ham towers/antenna farms have a number of antennas in close proximity and add to that antenna supports, guy wires, neighborhood power lines, etc. and none of us have the equivalent of an RF anechoic chamber surrounding an antenna.

Depending upon the type of pecan trees, you may find they have brittle limbs and shed a lot of them during storms.  My parents and grandparents bought adjoining double lots in what was once a pecan orchard so I grew up around a lot of them.  Beautiful trees but one row in particular had very fragile limbs so they may not be the best for reliable antenna supports.

I can't imagine your setup generating any unusual RFI problems as long as all of your connections are good.  Out of my available antennas, my favorite for 75 and 40 meter stateside coverage is a full wave 80 meter loop I have around 40 feet in the air.  The only problem with it is my Direct TV dish is fairly close and running full strap on 75 causes intermittent loss of signal.

It is currently 7 degrees and windy here in IL, I realized I needed to check the antifreeze quality in my tractor but at least it is in a pretty warm barn.  It definitely isn't good weather for antenna work here.

Rodger WQ9E
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« Reply #2 on: January 13, 2009, 02:56:06 PM »

All I can tell ya Todd ol' pal is;

This fall I hungded a 40 meter diperpole off the west side of the tower, feed point even in height with the 75 meter ant. They were seperated by @10 ft and the interaction was horrendusioius.
So I put both ants on the same feedpoint and alls well. Fan dipoles with 160,80 and 75, common feed from your highest tree will work great. With a gooder tuna the 160 ant will be great on 40 !!!

BUT, i'M JUST A BUDDLY.... whatdoiknow?
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Steve - WB3HUZ
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« Reply #3 on: January 13, 2009, 08:15:50 PM »

You can tune/detune the 80 meter antenna when not in use and it will become transparent RF-wise (is this a word) to the 160 meter dipole. It may be as simple as shorting it.

Or.... as Buddly noted, you could hang the 160 and 80 meter dipole from the same feedpoint/feedline. I did this with my 80/40 dipoles at a the previous VA QTH with no problems.

http://www.amwindow.org/misc/newant/40End.jpg
http://www.amwindow.org/misc/newant/OneLeg.jpg
http://www.amwindow.org/misc/newant/WireSky.jpg
http://www.amwindow.org/misc/newant/FandipWorksheet.gif
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K1JJ
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« Reply #4 on: January 13, 2009, 09:37:25 PM »

Hi Todd,

It's good you're thinking about this stuff now before erecting the antennas... :-)

Generally, the 75M antenna will see the 160M antenna and could cause a "divet" in the 75M pattern.  However, the 75M dipole is invisible to the 160M antenna.

There's one good way to find out the exact interaction going on...  Dump 100 watts into the 75M dipole. Put a termination resistor (50 ohms?) across the 160M dipole's feedpoint connector and measure the RF voltage  across this resistor with your scope. P = e2 /R.  OR, use a sensitive wattmeter and measure the power induced into the 160M dipole at the coax connector.

Either way, lets say you have 1 watt coming out of the 160M dipole.  100 watts into the 75M dipole with 1 watt coming out of the 160M dipole means -20db of isolation.   Minus 15-20db is probably the minimum goal to shoot for, since this is the figure that begins to change the pattern of a good figure eight dipole at a decent height above ground.    It's too easy to lose a few db in the desired direction too, because of reflector action, so run the test and see.  The reflector (160M dipole) doesn't have to be resonant as a 75M reflector to give you 5db of f-b - something you do not want in the desired direction.

Worst case, as Huzman says, you can always detune the 160M dipole and watch results.


Another test is to dump power into the 75M dipole and watch the swr meter. Then short and unshort the 160M dipole's connector. If you see movement in the swr, then there is interaction going on to SOME degree, which means further investigation. If no movement, then run the other test above anyway, to be sure.

One last test: Put a small 75M beacon as far as possible in the desired direction. Tune the signal in on your 75M dipole and make an S-meter measurement. Then raise and lower the 160M dipole to see if there are any effects at all on the S-meter reading. I built up a one fet oscillator for the job that runs on a 9V battery and crystal controlled. For the beacon, throw up a horizontal wire. Try to have the beacon at least 300' away.


Now you have some active methods to quickly test interaction in the real whirl - and see the effects as you move the antennas around, detune and try different stuff.

Good luck, OM!

T
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« Reply #5 on: January 13, 2009, 09:57:58 PM »

Why not put up a pair of 160 meter dipoles feed with open wire line and phase them on both bands so you can strap our yankee asses.
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K1JJ
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« Reply #6 on: January 13, 2009, 10:03:00 PM »

Why not put up a pair of 160 meter dipoles feed with open wire line and phase them on both bands so you can strap our yankee asses.

Good point, Frank!

You reminded me that maybe Todd should try the two phased dipoles project. Now THAT is worth the while. He can obtain a sharper receive pattern off the sides and off the back -  to squelch out any evil alligators down there... :-)  And be BIG up North.


Two phased dipoles - Northeast or Southwest Or Cloud Burner:
http://amfone.net/ECSound/K1JJ21.htm

T
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« Reply #7 on: January 13, 2009, 10:30:50 PM »

So what is this Tom? A 2 element system with 90 degree spacing  and 90 degree phasing?
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K1JJ
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« Reply #8 on: January 13, 2009, 10:49:10 PM »

So what is this Tom? A 2 element system with 90 degree spacing  and 90 degree phasing?


Yep, the same system that Frank/GFZ and Gary/INR  are using.


Actually, the spacing isn't critical - could be anywhere from 40'-80' between dipoles. Also, the phasing can be adjusted (by coax length) to find the sweet spot which yields the maximum front-back on the air, using A/B tests agauinst a reference dipole.  Up to 120 degrees is OK according to modeling.

A few guys in the DX window run it and I think it's the best antenna when you are limited to ~60' high on 75M.  Beats a parasitic 2el Yagi at 60'.

It has "HUZ" written all over it, caw mawn.

T


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Steve - WB3HUZ
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« Reply #9 on: January 13, 2009, 11:06:53 PM »

Ok. I was wondering how it would work with random lengths of feedline to the shack. The odds of ever getting the required 90 (or yes, better according to the modeling at around 120) degrees phase shift are about zero. You can only connect the two feedlines together at a point where the voltages are equal and expect to get equal currents and the correct phase shift at the feedpoints of the wires.
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K1JJ
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« Reply #10 on: January 13, 2009, 11:21:26 PM »

Ok. I was wondering how it would work with random lengths of feedline to the shack. The odds of ever getting the required 90 (or yes, better according to the modeling at around 120) degrees phase shift are about zero. You can only connect the two feedlines together at a point where the voltages are equal and expect to get equal currents and the correct phase shift at the feedpoints of the wires.

Yep, that's correct.

The best method is to first match the feedlines as near to 1:1 as possible for each antenna. This way, you have a constant impedance to work with, no matter the feedline length.  You want two identical antennas, too.

A good practice is to make the feedlines equal length and at a CURRENT forced-fed point.  This maintains phase better than at a voltage/high impedance point. A half wave multiple coax length is in order. Say, 86' or  172' (includes 0.66 velocity factor) from each dipole, whatever is easiest. THEN add on your 90 degree long phasing length to either dipole, depending on the direction desired.

The best way is to start at 90 degrees (or less) and play around with a bunch of 3' to 5' coax jumpers. You will find a point where +- 5' coax length or so gives you a good 15-18db front to back.  That is the sweet spot for a particular frequency range.

Later -

T
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« Reply #11 on: January 13, 2009, 11:35:20 PM »

What angle did you maximize the FB for? As you change the phasing, you can trade off between low angle FB and higher angle FB. For the AMer, phase angles around 140 improve the higher angle FB several dB (better for stateside noise/QRM rejection), but you start to get a lobe of the back at around 30 degrees. The FB there is still more than 10 dB. And the larger phase angle also lower the forward TO angle several degrees (more than several from the 90 degree case - from like 60 to down around 48 degrees. ON4UN usually recommends optimizing the FB at 25 degrees, but he's focused solely on DX.
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K1JJ
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« Reply #12 on: January 13, 2009, 11:42:55 PM »

What angle did you maximize the FB for? As you change the phasing, you can trade off between low angle FB and higher angle FB. For the AMer, phase angles around 140 improve the higher angle FB several dB (better for stateside noise/QRM rejection), but you start to get a lobe of the back at around 30 degrees. The FB there is still more than 10 dB. And the larger phase angle also lower the forward TO angle several degrees (more than several from the 90 degree case - from like 60 to down around 48 degrees. ON4UN usually recommends optimizing the FB at 25 degrees, but he's focused solely on DX.

Sounds like youse gots the modeling program out... :-)

Well, I say adjust the phasing for the best f-b at  the same angle as your forward lobe. So if the ant is at 120' high, (30 degrees take-off) then shoot for a minimum rear lobe at 30 degrees.   An antenna at 60' high is going to be dealing with higher angles off the front and back. You want to focus on the most energy to work with, and those are the areas, dictated by height above ground.

As you have found, lengthening the phase is like lengthening the boom to make the T.O. angle drop lower. I like to run the phase delay as long as possible for the sharpest pattern and make the array come to life..

For cloud burner operation, you probably already see that tying equal length coaxs together (zero phase) will give the could burner local coverage.

I know Gary made up an elaborate switching box for coax that gave him NE, SW and Cloud coverage. It can be simple or more complex.

T
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Steve - WB3HUZ
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« Reply #13 on: January 14, 2009, 12:04:17 AM »

Yes indeed. Az and el for the array with 130 degree phasing, 60 feet above good ground, compared to a dipole at the same height. Design freq is 3.8 MHz.




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Todd, KA1KAQ
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« Reply #14 on: January 14, 2009, 10:25:10 AM »

I would LOVE to go this route. Having heard and seen (via the Guess meter) the difference in Gary's signal when he switches the pattern(generally 15-20dB), it really looks like a case of 'best bang for the buck'. I think the max spread I could get now is only 25 feet or so, though. We got this place as more of a temporary, stop-gap measure and hadn't really thought of it as where we'd stay for any length of time. If we decide to stay put, I'll likely get some poles sunk and give this a whirl. Always wanted to run balanced feeders, was just talking with K4OZY about it yesterday, in fact.

Meantime I will give things a look once the ants are up. Sounds like a simple enough way to check, and I can decide how high to hang that end of the 80m dipole if it's an issue. The main idea is to get the 160 up as high and flat as possible and work from there. I was tending to think as Rodger said, that I'm worrying too much about too little. But with some of the best antenna gurus on amfone, just hadta ask.

BUT, i'M JUST A BUDDLY.... whatdoiknow?

And you're the BEST Buddly I know! The only one too, but hey...   Grin

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« Reply #15 on: January 14, 2009, 11:03:50 AM »

Yes indeed. Az and el for the array with 130 degree phasing, 60 feet above good ground, compared to a dipole at the same height. Design freq is 3.8 MHz.


Now THOSE are beautiful patterns, Steve.

It's one of those anomolies in life where you get something for almost nothing. To duplicate that pattern in other modest antenna configs would be difficult.  A Yagi needs more height to parasitically couple that well and a vertical needs great Earth ground and a ton of radials. This antenna performs at 60' high and with no ground at all.

Anyone who plays with a combination of local, and directional DX on 75M and has the room to put up two dipoles spaced 40-60' apart at 60' or higher would do well trying this out.

Later -

T
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« Reply #16 on: January 14, 2009, 11:14:49 AM »

Another option a Vee Beam 250 feet on a leg facing NW. I had a 2 wire Vee beam when I was a kid that burned a hole in VK land
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Steve - WB3HUZ
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« Reply #17 on: January 14, 2009, 11:21:51 AM »

Keep in mind you can get just about the same performance with as little as 32-33 foot spacing. You'll just need to adjust the phasing accordingly. The downside is that the close-spaced system is a little less broadbanded.

Another one to consider is a capacitively loaded Extended Double Zepp. You get almost as much gain as the phased-array, it's bidirectional and there is no phasing or switching required. The best part is that you can feed it with coax and it will have a SWR less than about 2.5:1 across the entire 80/75 meter band (well less than 2:1 ovee most of it)! Sounds too good to be true. Check out the article by N6LF.

http://rudys.typepad.com/ant/files/antenna_colinear_zepp.pdf
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« Reply #18 on: January 14, 2009, 12:13:52 PM »

Another one to consider is a capacitively loaded Extended Double Zepp. You get almost as much gain as the phased-array, it's bidirectional and there is no phasing or switching required.

Bi-directional... that's the bug-a-boo... :-)  You know all this already but to repeat for others: On 75M the receiving advantage of using a unidirectional antenna is tremendous. Being able to cancel out a complete hemisphere of atmospheric noise behind you really makes the difference between hearing what everybody hears and hearing what the majority cannot.

Axe Chuck/K1KW what happened when he went from his 75M 3el Yagi at 100' to a pair of half waves in phase (fed with openwire) at 150'.  The gain was about the same and the two half waves even had a slightly lower TO angle. BUT, he said he couldn't hear for dick. The atmospheric noise off the back (not QRM) was so bad that he gave up and rarely got on the DX window cuz he claims he couldn't hear with it into Europe. In contrast, he had some of the best ears on the band with the unidirectional Yagi. I've experienced the same thing here.

So, bottom line, that unidirectional pattern you posted is worth its weight in gold. To maintain forward gain (without ground losses or T.O. angles that are too high) is really doing something at only 60' high.  Your model is showing -25db front to back - beautiful receiving antenna.  (-20db f-b vs: the dipole ref) Imagine adding a 600' terminated beverage in the other ear. (diversity system)

BTW, speaking of two half waves in phase: I designed up a system for Bob/ZM that uses a pair of reflectors behind it that can be vac relay switched to directors using stubs. Unidirectional. His system works like a bomb. That's the only way to do it right if ya wanna hear well.

T
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Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

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« Reply #19 on: January 14, 2009, 01:03:37 PM »

25 foot spacing will work but the input Z will go down. I've always wanted two dipoles fed with open wire line to be usable over a wide frequency range.
Back to Todd's reality how about a 2 wire dipole with the ends fanned out fed with open wire line to work all bands. K9AY for RX.
It would be cool to have Tom build a k9ay and compare it to the other antenna farm animals.
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« Reply #20 on: January 14, 2009, 01:20:14 PM »

K9AY for RX.
It would be cool to have Tom build a k9ay and compare it to the other antenna farm animals.

Others have already done the tests.  On 75M, when a terminated, full size Beverage is compared to a K9AY, the beverage usually gives a better signal to noise ratio.  When the beverage is compared to a high, well tuned Yagi, the Yagi is usually slightly better than the Bev, depending on condix.

The K9AY and other small receiving loops are a wonderful compromise for space limited QTH's. However, just like any antenna, they need to be in the clear.  Some work poorly cuz they are mounted next to houses, etc.

Especially the Beverage, if there are any other antennas or even towers in the near vicinity, their patterns go to heck and couple into the surrounding structures.

I've had a SE Bev run thru the tower field. Never cud get it to receive well. After moving it out into the woods about 200' away from the towers, in came to life and has become an official member of the family.. :-)

T
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« Reply #21 on: January 14, 2009, 03:34:09 PM »


It would be cool to have Tom build a k9ay and compare it to the other antenna farm animals.


Check out this   http://www.hard-core-dx.com/nordicdx/antenna/loop/k9ay/k9ay17.html

Pretty decent comparison of beverage vs k9ay setup..  He even uses the k9ay and the bev together at one point, testing them.

I haven't finished reading the article, but thought I'd pass this along. 

--Shane
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« Reply #22 on: January 15, 2009, 05:07:06 AM »

I have a 585 ft bev that has its feed end under one leg of my 60 ft hi 80 M dipole.....It is usually the only bev in my farm that   gets noise on it...Not always and never too much noise...It is aimed at California and is fed with a 9:1 xfmr and terminated....I've always wondered if it was interacting with the dipole somehow.
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« Reply #23 on: January 15, 2009, 11:22:57 AM »

I have a 585 ft bev that has its feed end under one leg of my 60 ft hi 80 M dipole.....It is usually the only bev in my farm that   gets noise on it...Not always and never too much noise...It is aimed at California and is fed with a 9:1 xfmr and terminated....I've always wondered if it was interacting with the dipole somehow.


Hi Steve,

Over here, I've found that certain Beverage directions have noise a higher percentage of the time than others.

For example, my NE and SE Bevs are usually very quiet. But the west Bev is usually noisy. They are all way out in the woods away from any metallic influences. In my case, it's atmospheric noise propagating thru skip, not local line noise. It may have something to do with the largest land mass (USA) is to my west and the two other directions are mostly water for the first skip zone.

There's a lot of stuff on land, like houses, factories, power lines, etc, that can cumulatively add up to a skip noise signal.  Each QTH will have different sources of noise, both skip and locally induced, of course.  The way to know if it's skip and not local, is to watch the noise level increase dramatically during and after sunset. (Or the opposite at sunrise) You may see it go from S1 to S9 as the local noise gets buried by the skip noise.

The sure way to find out if the 75M dipole is causing your problem is to make a careful S-Meter measurement with the noise/west Bev and then drop down the dipole to the ground (lying on the ground) and see the difference, if any. If it's the dipole causing the interaction, the obvious solution is to shorten then beginning of the beverage feedpoint by 100' or so. That will have little effect on the 600' Bev. (Or lengthen it on the other end to compensate)

Good luck -

T





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And, nothing like an old dog.
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« Reply #24 on: January 19, 2009, 07:48:53 PM »

For sure the unidirectional antenna will be the better on receive. I posted the EDZ because it trades off wide band width and simplicity for the loss of unidirectionality. The phased array will not maintain good gain and F/B over the entire band. The EDZ will maintain its gain over the entire band (only 1 dB less than the array at its best freq), and with low SWR. And it's a much more simple antenna to install. If he feeds it with open wire he could use it one 160, a two-for-one. This does not make it better, just pointing out the trade-offs.

For unidirectionality on receive a beverage or terminated loop will do very nicely and will easily beat a low phased array (especially a beverage with its high lateral directivity). And a properly installed beverage will always beat any of the terminated loop designs (EWE, K9AY, etc).

For AM use, Todd would probably be better off with one of the terminated loops since they have better high-angle response. The attached sound file is just one example of how the K9AY helps on receive at HUZ Radio. The clip is of Gary - W1GHW out of Fairfield, CT on 160 meters at around 10 PM ET in December 2008. To be fair to Gary, he usually puts in a much better signal on 160, but on this particular night, his signal was not strong and was subject to fading on the dipole. On the K9AY, he was armchair copy.


Another one to consider is a capacitively loaded Extended Double Zepp. You get almost as much gain as the phased-array, it's bidirectional and there is no phasing or switching required.

Bi-directional... that's the bug-a-boo... :-)  You know all this already but to repeat for others: On 75M the receiving advantage of using a unidirectional antenna is tremendous. Being able to cancel out a complete hemisphere of atmospheric noise behind you really makes the difference between hearing what everybody hears and hearing what the majority cannot.

Axe Chuck/K1KW what happened when he went from his 75M 3el Yagi at 100' to a pair of half waves in phase (fed with openwire) at 150'.  The gain was about the same and the two half waves even had a slightly lower TO angle. BUT, he said he couldn't hear for dick. The atmospheric noise off the back (not QRM) was so bad that he gave up and rarely got on the DX window cuz he claims he couldn't hear with it into Europe. In contrast, he had some of the best ears on the band with the unidirectional Yagi. I've experienced the same thing here.

So, bottom line, that unidirectional pattern you posted is worth its weight in gold. To maintain forward gain (without ground losses or T.O. angles that are too high) is really doing something at only 60' high.  Your model is showing -25db front to back - beautiful receiving antenna.  (-20db f-b vs: the dipole ref) Imagine adding a 600' terminated beverage in the other ear. (diversity system)

BTW, speaking of two half waves in phase: I designed up a system for Bob/ZM that uses a pair of reflectors behind it that can be vac relay switched to directors using stubs. Unidirectional. His system works like a bomb. That's the only way to do it right if ya wanna hear well.

T

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