A Loop/Dipole Hybrid Antenna Array

[KK4YY]

A what did you say?
So, we use horizontal loop reflectors under horizontal loop antennas, and we use horizontal dipole reflectors under horizontal dipole antennas. How about a horizontal dipole reflector under a horizontal loop antenna?

I have a horizontal 75M full wave loop at 30'. A loop reflector 10' above ground would be a problem for me — it would be in everyone's way.  A straight wire, corner-to-corner, at 10' would not be in the way.

In the attached illustration, at first glace, it looks like the reflector is only 20' below the loop. And it is, but the reflector is 50' from the current maximas of the loop antenna — about 0.2 wavelength from them. Well, that's an interesting place to have a reflector.

Before I start stretching wire around the yard, has anyone seen this kind of arrangement before? Any ideas about how it might work? I'd be interested in how much it might reduce ground losses, and how it might effect the azimuth and elevation patterns of the loop.

Any ideas?


Don

[K1JJ]

Hi Don,

I have experimented with similar systems and found no difference on 75M for distances from 30 - 600 miles.  

But we can speculate 'til doomsday. The real accurate way to find out at YOUR QTH is to put up the reflector and place relay contacts at its center with the relay 12V control wires run straight down to the ground and into the shack. Make or break the connection to become a ~ 1/2 reflector or invisible 1/4 wave wires.  Do lots of AM carrier A/B receive tests watching the S meter and it will tell you the story.  Despite all the NVIS talk, I'll bet after trying a lot of configurations and reflector positions you will see no difference.   I've tried and never could...  

BTW, I did another experiment dumping a few hundred watts into the dipole. I walked along the ~130' reflector 10' off the ground and was able to light a florescent tube quite brightly. Despite this encouragement, I saw no difference in 75M receive signals when doing A/B.  It was definitely coupling and resonating because when I opened the center connection, the light went out.

I also tried remotely tuning the reflector going from director to resonant to a reflector length - but still had no higher angle improvement.

T

[KK4YY]

Tom,

Thanks for you comments. I was hoping you'd weigh-in on this.

Your idea to put a relay in the reflector element solves the problem of A/B testing. I was wondering how I could make comparison measurements. That's doable here. Also, if a difference was noted, it could be left in-line and used during normal operations (assuming the relay can withstand transmit voltages).

The null in the pattern of my loop appears to be for very close-in stations, like 10 miles out. This is contrary to my expectations for a loop. If I could A/B the reflector and see a difference there (for better or worse), that would tell the story.

I didn't have high expectations for this concept and your experience with similar arrays hasn't brightened the outlook any. But, it's easy enough to try, so I'll put it on my projects list for this Summer. It should make for a fun afternoon either way.


Don

[K1JJ]

Hi Don,

Some more thoughts:

Assuming you can prove a reflector works for you with the initial high angle A/B testing, probably the best way to cover the various lower take-off angles is to put up many reflectors extending away under the loop/dipole. (reflector screen)  Picture yourself climbing the tower to the dipole and shining a flashlight down onto a huge mirror on the ground that extends out 600' around you.   You can visualize how far away the reflectors need to be to cover the very lower angles.   Obviously a single reflector directly under the dipole will not cover the valuable angles on 75M under 50-60 degrees which are very common at all hours of the day and points in the solar cycle.  

I've found that my 75M 2el delta loop array at 190', which has a nicely contained TO angle of 28 degrees, works best for TimTron in the day who is 300 miles away. That's why a dipole at 80-90' is a great compromise. A reflector screen under a 90' high dipole to cover 28 degrees TO angles would extend about 180' away from the center. The higher the antenna, the bigger the screen requirement to cover a given TO angle reflection.

Also, you will lose the RF ground depth on 75M that can give you an extra 15-20' of effective average height. IE, a ground screen locks in your height above ground. If you are struggling to get the antenna as high as possible due to support limitations, then maybe no reflector at all is best for the lowest TO angle as possible. (though maybe FB for NVIS work)  With a high tower, no problem.


BTW, re: your question about using a 1/2 wave parasitic wire element with a quad loop - it will work FB. They do this for "Quagi" arrays on 2M and get good coupling. (using directors)

T

[KK4YY]

Tom,

Yeah, I'm in a renting situation and a ground screen wouldn't work here. I can squeak in the one reflector - that's about it. I'm primarily interested in NE regional work so I'm not even trying to get a low TOA. If DX stations can't hear me, and I can't hear them, all the better. 

To that end, I've been thinking more about the reflector wire. Instead of a relay at the center, I could feed it with an electrical half-wave line and short/open the other end in the shack. Also, I could feed it directly as a true NVIS ant. I can tune/trim it with the rig and SWR meter as well. There's a few combinations there - receive on one transmit on the other, etc. This project is getting more interesting.

Yes, the quagi ant. It had crossed my mind. This may be a kind of reverse quagi of sorts. But, a parasitic element is a parasitic element, I suppose.

There's been a lot of conjecture over the many years regarding gain of loops vs dipoles. I've landed on thinking that the difference is too small to argue about (but some still argue). To me they're different in other, more significant, ways. One may fit in a particular physical space better than the other, and operation on harmonics differs. It's pretty easy to make a 75M loop operate on 40M with a coax feedline, as I've done here at my apartment. At my camp, I have 75M and 40M dipoles fed with a common coax feedline. Works like a charm, and that's what fits in the space that I have there. At my apartment, I can get a loop higher than I could get a dipole (for 75M) but I think I can fit a low dipole here, hence the reflector wire idea.

At this point my interest is at a high level. So, one of these nice weekends...


Don

[Steve - K4HX]

How do you know there is a null in the pattern? It could be cancellation of the ground wave and skywave.

The null in the pattern of my loop appears to be for very close-in stations, like 10 miles out. This is contrary to my expectations for a loop. If I could A/B the reflector and see a difference there (for better or worse), that would tell the story.

[KK4YY]

How do you know there is a null in the pattern? It could be cancellation of the ground wave and skywave.

The null in the pattern of my loop appears to be for very close-in stations, like 10 miles out. This is contrary to my expectations for a loop. If I could A/B the reflector and see a difference there (for better or worse), that would tell the story.

Steve,

I would consider that type of cancellation also to be a "null". But perhaps I'm using the term too broadly.

Here's the exact situation that has me perplexed. I'm located 10 miles from Bear, WB2GCR and we typically have very poor signals to each other on 75M during the daylight hours. At night, no problem.

He has a Valiant into a dipole, I have a DX-100 into a horizontal FW loop. With both antennas having a predominant horizontal polarization, I wouldn't expect a very strong ground/surface wave on 75M at that distance (I would think it to be too far). Whatever ground wave does exist, it would be there regardless of skywave propagation. If the two are cancelling, why only during the daytime? To get cancellation the amplitude and phase need to be "just right". With the vagaries of skywave propagation, I wouldn't expect things to line-up like that very often.

So, I'm still puzzled. All I can do is try different things to figure out what's going on. Maybe with the very low dipole antenna which I proposed earlier, some light will be shed on this.

I am, however, comforted by the fact that if nothing that I do works, I can always blame his antenna.


Don

[KD6VXI]

What is the s meter reading during the day?  At night?

Same with noise level?

--Shane
KD6VXI

[Steve - K4HX]

Very interesting. I'd blame Bear's antenna. 

I'd say it's not a null in the antenna pattern since it's not there at night. It's likely your skywave propagation in the day is off the E-layer. At night it will be F-layer. So the geometry changes. But I wouldn't expect a null any time since you both have horizontal antennas. Better ground wave propagation is achieved with a vertical radiator. Maybe direct wave?

[K1JJ]

Don,

I would say the weak signals between you and Bear are simply a matter of the optimum layer absorbing most of your signals in the day.  One way to find out is to find another ham that is 10 miles away in another direction and watch his signal at the same time.

A straight up angle may be very lossy during the day whereas a lower angle thru the same layer may be less lossy for farther away stations.

I also believe on 75M the groundwave distance is very short, probably less than 5 miles.   W1AW is about 15 miles from me and I see their signal very weak in the day and strong with fading all throughout the night, showing that close-in sky prop is at work on 75M.  And, watch how Tron's mobile signal suddenly goes crazy and climbs rapidly and stays solid as he nears your QTH during a visit....  

Possibly the day absorption is so high that even a few dB gained from an NVIS antenna would have little effect.

That said, I have experienced extreme fluctuations in daytime signals and you may occasionally see the effects too.

** One time in 1973 a friend and I who lived about 2 miles apart  went home for lunch (12 noon)  and got on 80M CW for a test. I was amazed to see his signal rapidly fluctuating between S4 to 59 +60 over!  It was a rapid, wild fade every second.  This went on for the 15 minutes we were on the air. It was like someone was turning the RF gain up and down on the receiver.  We concluded that we were at the perfect distance apart (2 miles) for the ground wave and the lowest layer to cancel. IE, his signal would have normally been S9+60 via groundwave, but the layer phasing nulled it way down at times.

This may not be your situation, but the bottom line is you need some more stations to get involved in testing close by to prove what is really happening.


BTW, speaking of strange propagation... has anyone ever experienced LDE, long delay echos on 75M?   When I lived in Colorado in the early 70's, I had up a dipole at 70' with 2KW. I was in a local QSO on ssb in the evening and suddenly heard my voice echoed back to me about 1 second delayed. The other guys in the QSO were not heard, just me.  It went on for about 60 seconds and then stopped. A trip around the Earth would be 1/7 second, so this was something else.  I heard it again in 1990 on 75M in the DX window. I was running a Yagi with high power and could hear my own echos. This may have been a trip around the Earth, I dunno, because it had a very fast response.  Strange brew.

Ever since I started wearing a tin foil hat.....  I seldom hear those things now.

T

[W2NBC]

" and could hear my own echos. "

Hmmm

[K1JJ]

Too funny!

Lucy in the Sky with Diamonds?

While we're at it....   check this out:
https://www.youtube.com/watch?v=QjvzCTqkBDQ


INFO on Long Delayed Echo....
https://en.wikipedia.org/wiki/Long_delayed_echo


T

[PA0NVD]

T
[/quote]BTW, speaking of strange propagation... has anyone ever experienced LDE, long delay echos on 75M?   When I lived in Colorado, I had up a dipole at 70' with 2KW. I was in a local QSO on ssb in the evening and suddenly heard my voice echoed back to me about 1 second delayed. The other guys in the QSO were not heard, just me.

I once heard a portugese  station when living in Spain that had a approx 1/3 of a second echo. He was pointing west. When he changed antenna to the north, it was gone. I also considered that it must have been an around the globe effect It was at 14 MHz.

[KK4YY]

S meter readings...  I'll have to check. My S meter varies with the RF gain control so I'm not usually looking.
There's not too many stations using AM close-by to do a lot of testing. As far as I can recall from nearby stations during daylight, It goes like this:

1 mile - good
4 miles - good
10 miles - bad
23 miles - good

Pretty unscientific, but that's all I have for now.

Maybe 99% of stations on 75M are horizontally polarized as well as not having strong, extremely low take-off angles. Those two factors may explain why ground/surface/direct wave propagation is so very short - there isn't much power there in the first place, and what little there is, gets quickly shunted to ground.

So that leaves a blind-spot of say, 5 to 15 miles, where there is a lot of D layer absorption and little refraction due to the near perpendicular angle of incidence with the ionosphere. At night, as the D layer dissipates, and the more refractive F layer comes into play, signals are again at an acceptable level.

This is all conjecture, but it's difficult to make good measurements. I may strive to understand it, but in the end I'll probably just have to live with it.


Don