Recommended distance between 2 Antennas

[W2INR]

I have a 10 -20 meter 3 element beam and a 5 element 6 meter beam I am going to put up in the very near future. What is the preferred distanced between these two antennas and then the closest one should bring these together with or without compromise to the patterns?

Both antennas will be mounted horizotal.

G

[WA1GFZ]

I would think at least 1/4 wave on 20 meters but a 1/2 wave would be better.

[WBear2GCR]

 Is that a tribander and a 6m beam?

Or a 10, a 20 and a 6?

The usual thing is to put them up xmas tree style.

Afaik the distance between them in the vertical plane is not terribly significant, but one could presumably model it in EZNEC or similar and see the presumed effect. My limited knowledge which is likely to be wrong is that a 6m beam positioned 6ft or so above a 20m beam will only add a bit of capacitance, and mildly alter the match. Perhaps even help reduce some radiation that would otherwise go vertical?

I'd throw it up and not worry much myself.

                   _-_-bear

[K1JJ]

G,

The 20M portion of the  tribander beam will not see the 6M beam cuz the 6M beam's elements and boom are shorter than 1/2 wavelength on 20M.

The 15M portion will be OK too, cuz I think your 6M boom is shorter than 22' now.

However, the 10M portion may have a resonance problem with the 6M beam depending on boom length and the shorter 6M directors.... but to a degree.


The 6M beam will have a tendency to see the TriBander much more and is the main problem.


One way to help reduce interaction to the elements is to mount the beams at right angles to one another. You then need to mentally figure  a 90 degree pointing correction using the 6M beam.  There will still be SOME interaction between the beams, no matter what orientation or stacking distance you use.  The booms are a different matter and depend on their length and the loading effect the elements have to make them electrically longer.

Modeling will show that when two dipoles are placed at right angles, there is little to no interaction... however, when elements are added to one or both, then interaction can occur.

It's really a matter of degree and how anal you wanna be. If it were me, I would mount the two antennas at right angles AND stack them as far apart as mechanically possible. I'll bet you will have a pretty clean pattern on all bands with some luck.  Ten foot spacing, and placed at right angles would probably give minimum interaction and reasonably transparent patterns.

Once on the air with the new installation, you can short out the feedline of one antenna and watch the swr on the other using the MFJ-259 ant analyzer. There's other ways to check for interaction too, like putting power into one and measuring the other with a sensitive power meter at the coax connector...   This will give a relative idea only, cuz each antenna is designed for a different freq and the interfering elements may not transfer energy efficiently to its own driven element - thus, will greatly affect the results.

If the two antennas were of the same band, then 100W into one and 1 watt out of the other means a -20db isolation, which is a reasonable number for minimum interaction.

Later -


T

 

[WA1GFZ]

Tom,
But now the boom is parallel to the elements at 90 degrees.
I used to have a 3 band quad with feed lines on each band. There was plenty of interaction. When we got warc bands I figured it wasn't worth the trouble and went with the LPDA. Also a famous user told me it would work on 6 meters even though cut for 30 MHz tops because the 3/4 wave elements would take power. Gary has enough metal to build one antenna.
But I would keep the 6 meter ant 1/4 wave away from the 20 meter ant as a minimum. You know there will be some interaction at 1/4 wave on 20.
I R no tenna spert though.

[K1JJ]

Tom,
But now the boom is parallel to the elements at 90 degrees.

Frank,

Yep, I partially mentioned that above, but is worth expanding upon....  The advantage of having the boom in the field is that the boom has only one length to deal with, whereas, the elements and trap resonances are all over the place.  Easier to deal with just the boom, I think.

Yes, an LPDA wud be a great alternative, but only for 20-10M.  Adding 6M to the LPDA is pushing it and a longer boom Yagi like G has now is a much better antenna.  Also, I don't think G's supporting structure would handle an LPDA like yours... a light tribander and 6M beam is about it.

I imagine he will mount the Tribander at the top of the tower and extend a smaller diameter mast above that as long as possible for the lighter 6M beam.

T

[WA1GFZ]

Plug a 20 foot pipe into the top of the tower and mount the 6 meter and at the top. Then slide the pipe up and attach the other ant. Put the rotator inside the tower.  Yea my ant has 25 pretty much maxed out. That is why I have 4 sets of guys and anti twist brackets for 70 feet of tower.

[K1JJ]

Yeah, the 20' pipe would be a great idea.

However, it's really an aluminum? (I think) 6" diameter? 70'? military mast clamped together in 5' sections.  It's quite a mechanical wonder and has a semi-automate raise and lower arrangement. 

But it's no 25G, I'm afraid.

T

[Pete, WA2CWA]

I have a 20 foot pipe sticking out of the tower. At the bottom, a five element, 26 foot boom, tri-bander is mounted; 8 foot above that, the 31 foot boom 6 meter beam; 8 foot above the 6 meter beam is the 31 foot boom 2 meter beam. Using the MFJ259 shows no interaction between any of the beams on any of the frequencies in question. Have it this way for almost 20 years. This summer, I'm going to slide the 2 meter beam up and mount a 432 beam between it and the 6 meter beam.

Even if you dropped the 6 meter beam so it rested on the boom of the tri-bander, your E-H pattern of the 6 meter beam might only change by a few degrees. I doubt you would see any change in tri-band performance.

[W2INR]

Thanks for the input.

T , you are right. I am using the Ab-577 military mast system which gives me 50' stock and I have added 4 sections which  gives me a height of 70'.

I have engineered a way to mount a HAM 4 and a thrust bearing on the tube and this will allow me the opportunity to use a 20' mast ( which would be over a 1/4 wave on 20 Frank ) but - - what type of pipe am I looking at here. The only weight on the entire pipe will be the 6 meter beam and the tri bander will be sitting just above the thrust bearing.

Heres a shot of the stock ab 577

G

[K9TR]

F.W.I.W when I put a 3 element 6 meter yagi 8 feet above a TA-33 3 element tribander, I noticed minimal interactions.  Nothing really shifted to speak of on the impedance of the tribander and the 6 meter yagi worked as advertised.  A 5 element 6 meter yagi is presenting more aluminum for your tribander to stare at but I would imagine if you can manage 8 to 10 feet of spacing (put the 6 meter yagi on top, save your rotor ) things should work out just fine.

Mark K9TR

[K1JJ]

Even if you dropped the 6 meter beam so it rested on the boom of the tri-bander, your E-H pattern of the 6 meter beam might only change by a few degrees. I doubt you would see any change in tri-band performance.

Yep, I hear ya, Pete.

It's really a matter of what you are looking for and your standards.

I feel that if you tune a Yagi for maximum front to back, (30 db or more)  it is very sensitive to the surrounding environment and that f-b can be easily snuffed back down to 15db f-b or less. It's a very high Q circuit. 

All we really have in this world is front to back -  and front to side as our receiving edge. The forward gain (which is less sensitive to interaction) gives us little advantage.  Here's why:

Take a standard 3el trapped Tribander. First, the forward gain of a FULL-SIZE  3el Yagi over a dipole is about 5db.  Now the tribander: The trap loss is as least 1db. (or more)  The feedline loss is maybe 1/2 to 1db on the higher bands. What does this leave for forward gain?  If a station is fading up and down 10-20db, what difference does a few db forward gain make?  For either antenna, the forward gain is small.

However, the difference between a modest front to back and a tremendous f-b makes playing the DX game worthwhile. There's nothing like a sharply tuned Yagi optimized for front to back to give a receiving edge = fun. Less atmospheric noise and qrm.

But what happens to our last edge (f-b) if we place the antennas too close together? I say if possible, space them apart as far as possible. If you can't, then you can't.  If it is controllable, I say do it.

I have run rotation tests with 6M Yagis that were optimized from 50.0 - 50.200  for very high f-b.  It amazed me how easily this f-b died when placed near other antennas. Even the reflector element moving within 2' of a tree branch caused a big change.  This will not be as dramatic with a broadly tuned Yagi covereing the whole band, of course, just for highly optimized antennas. I originally designed Gary's antenna for 50-50.2mhz and it was a sharply tuned design - that's why I wanted to see minimum interaction potential.

Of course not everyone has the desire or means, but a separate tower for each band is the ideal.   Or a log periodic covering a large spectrum. But you couldn't give me one of those densely stacked Yagi, "Sky Needles"  with 15 different beams installed for FREE. No thanks.

It's all a compromise and most any antenna will work FB. It's just a matter of your performance standards. When it comes to performance, I am cursed to being a perfectionist. If it doesn't work to it's best possible performance, I work on it until it does or rip it out and start over.... sigh...

Here's a shot of my homebrew 10M stacked array. These Yagis were designed and modeled together as a stack to compensate for mutual interaction. Even the guys nearest the Yagis are broken up into smaller than 1/2 wave 10M lengths. Still, it's not perfect. Single tower, single band. I practice what I preach...sometimes.
http://home.comcast.net/~k1jj/wsb/html/view.cgi-photo.html--SiteID-3018591.html


Gary:  About the mast/pipe question.... how long is the 6M boom after the crash?  Remember that you will need TWO support/collar points for the rotator/mast (like two thrust bearings spaced 3' apart or equivalent) to handle the side thrust stress on the rotator if the pipe very long.

T

[W2INR]

Ok T

I can add another bearing for strength.

The boom is around 12' long now. I do not recall the exact measurement.

G

[K1JJ]

Ok T
I can add another bearing for strength.
The boom is around 12' long now. I do not recall the exact measurement.
G

OK G -

Use one pipe for the whole thing. ~ 2" inside diameter is common. I use schedule 40 steel pipe for most everything cuz it's so plentiful at the junk yards.  I used to pay 30 cents a pound and drive away with a truck bed full of 10' lengths for $100.  It's somewhat higher now....

But for your installation, the tribander will be less mast stress cuz it's mounted right at the tower top and within a inch of the rotator.  The 12' boom 6M Yagi is pretty light too. So I wud use something in-between ....like schedule 30 or equiv (if it exists).  I'm not sure if schedule 20 is pushing it, but it may be OK too, as long as it's steel.

Stay away from aluminum masts unless you get a thick walled 1/8- 3/16" or so, but they're expensive.   1/8"  thick may not handle your heavy ice loads, but might squeak by OK. 

I would stop by the junk yard or metal recycling place and have a walk around. Your intuition will tell you what to use.

T

[WA1GFZ]

I used 2 inch thick wall aluminum out the top of the tower with dual TBs. Then I slid smaller pipes inside until the ID was about 3/4 inch. The tower would bend first.  Maybe you could make the section at the bottom dual wall pipe then kneck down at the top section to hold the smaller ant.