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Author Topic: HIGH SWR Tribander  (Read 27341 times)
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flintstone mop
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« on: July 30, 2005, 04:13:39 PM »

I am waiting for a reply from Cushcraft for this problem, but I would like to pose a question to the board also.
I set the measurements for the middle of the band for 20M, 15M and 10M. 10M is about 3:1 SWR near the AM Window 29.100
The Driven  element becomes a rotating dipole for 40M and I set this up for 'middle of the band' also and I discover that I  have 3:1 SWR at the high end of 40M, right at the AM Window. DAMN!!!
You don't find any of this crap out until it's off the ground.
My question is....Will it hurt or destroy any of the RF components (traps) running 200 watts AM??? That would translate to 800 Watts P.E.P.

Thanks

Fred
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« Reply #1 on: July 30, 2005, 08:55:03 PM »

Hi Fred,

That's a good question.  I'm not absoultely sure, but will take
a shot. At first I thought yes, a high swr would be a problem,
but after thinking about it, maybe not.

I guess the question is, if you swept that trap Yagi from 14mhz to
30 mhz continuos - with 1500W, would it do trap damage?

I would think that the maximum voltage across each trap would
occur at its tuned resonance spot, being a parallel tuned circuit.
And, at freqs other than resonance the trap would have minimum
voltage across it. So, generally a high SWR caused by being off
freq is not a problem, voltage arc-over-wise..

Also, since a mismatched feedline puts less power into the load,
you will have another buffer when off freq.

I think the problem is that even though the difference between
a 3:1 and 1:1 swr is probably not noticable on the air unless
you have a VERY long feedline, your radiation pattern could
go to hell.  ie, trap Yagis are inherently more narrow than
full-sized Yagis, and both the driven el and parasites use
similar designed traps. So, if you are seeing a 3:1 swr on
one end of the band, chances are that the reflector and
director are affecting the pattern enough to make your
front to back poor.

Do you have the published SWR curves and F-B curves
to compare against?  The f-b is easy to determine with
a turn of the beam. Make sure everything is working as
designed.

But, to answer your question, NO I don't think a 3:1 swr  
[or even higher]  will hurt anything on the beam, if my
assumptions above are correct.. Esp at only 800W pep.

Maybe some other guys here will have opinions on the
things discussed above.

Please let us know what the manufacturer says when they
get back to you. You've got me curious.

73,
Tom, K1JJ
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« Reply #2 on: July 30, 2005, 09:17:58 PM »

Fred:

Do you have a MFJ 259B or grid dip meter?  If so, have you swept the antenna to see if it dips and reasonates somewhere outside the bands?  

Am not sure how the feedline is matched to the elements, gamma, balun or what.  I had a similar problem and found the stuff I used to seal the coax at the split for the feed was causing a problem.  I cut the feed end off and remade it, then used something that was not reactive at RF and mine (not a Cushcraft) came into the band.
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« Reply #3 on: July 30, 2005, 11:24:31 PM »

Following up on Jim's comments...

Fred, what does the swr read in the middle of the band where
you set it? ie, are you seeing near 1:1 there, but are not happy
with the sharp curve?

Even my HB 3el 40M full sized Yagi with a 1:1 swr in the middle
of the band, I see the swr creep up to 2:1 on the band edges.  

Check the published curves against your actual readings if the
center readings are good.

T
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SWR
« Reply #4 on: July 31, 2005, 10:07:53 AM »

Thanks guys,
Thankfully I did not use anything on the connection to the driven element. I did spray a non conductive coating of a Plastic spray on the insulator only. There is 1 inch of no spray from the insulator and the actual driven element parts. And I sprayed this plastic insulator ONLY on the screws where the connection is made to the driven element. So, I don't think there's some weird RF leakage path in what I have done. This was done on the recommendation of Ed Hammond, (tech help at Cushcraft) before I had the antenna mounted on the pole. He actually suggested a varnish used by Marinas in case there was something conductive across the insulator to the driven elements.
My MFJ guy is telling me that the antenna is responding to the measurements for the middle of the band very nicely. 20M and 15M are very happy. 10M should have been set for the upper part of the band, I guess. It's just the sharp rise to 3:1 SWR for 10M and 40M that I'm concern me.
I'll confess that I do not have a connector on the loose end of the RG-213. I just jam the center wire into the Type-N female on the top of the MFJ and a clip lead for the ground. Maybe this is a nasty 50 ohm connection. It's been to blasted hot out there to put a PL-259 to do these measurements properly.
But I thank you for the responses and look for more from my remarks above.

Fred
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Jim, W5JO
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« Reply #5 on: July 31, 2005, 04:32:05 PM »

The correct connector will make some difference Fred.  I will be curious to know what you measure when you have it on.  Most of the difference may just be impedance.  Of course the 259B is designed for 50 ohms, it will measure out of that range but the readings are a bit off.
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« Reply #6 on: July 31, 2005, 09:51:49 PM »

Fred,
Try sweeping the ant to see if there is a low vswr point to determine if the resonance has shifted or there is a real failure. fc
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flintstone mop
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SWR
« Reply #7 on: August 03, 2005, 10:39:40 AM »

Hello all,
I received a response from Cushcraft about the 3:1 SWR on 10M and 40M and the following was all I got!

OM    the stingers at the end of the driven element must be peaked for the part of 40M    it cant be set at 7.050 for CW at the low SWR and then used at 1 KW  for AM   about 200 watts into a tuner  no problem      Ed Hammond Wn1i

Ed was helpful in the beginning about some questions on the install and the vague instructions for the 30/40M add-on, but the above response didn't help at all.

So, yous guys say that with 200 Watts AM there should be no melt down.

The only other thought that Ed didn't address in my e-mail was the alignment of the elements.

The director is not lined up with the other two elements.

Of, course this shows up when the antenna is off the ground. I was so beat from the things that went wrong with the install of the pole that I didn't notice this. The yard was a wreck from the big crane truck, and I was suffering from heat exhaustion. Believe me, I paid for it the next two days. It doesn't matter how much GATORade and water you drink. IF YOU ARE BECOMMING AN  OLD FART the high heat and humidity WILL get you.  And my wife wonders why I can't take the heat in the Philippines......haaaa haaaaa !

Thanks for reading the latest from MOP radio.
And the advice is always welcome. I think I'll have a good station back on the air in a couple of weeks. I'm still priming a dark green color in the 'shack' so, I can proceed with the install.....220VAC, grounding,,,lightning protect...........etc, etc

73     Fred
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Fred KC4MOP
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« Reply #8 on: August 03, 2005, 10:49:43 AM »

Fred,
You should have built a Log preiodic. I was not impressed with any crushcrap products when I was looking for an antenna. I made the mistake of buying a tennadyne T12 log. Very good electrical design but mechanical hunk of crap. I wouldn't let my kids play under it. I just about doubled the weight to make it strong enough. Tom Vu has built a couple logs and I bet he spent less money than I did. sorry om the lack of support means only 1 thing. Spread the word about th ecompany so nobody else gets screwed. Tell them of your plans Too.  fc
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« Reply #9 on: August 03, 2005, 11:21:24 AM »

Fred,
So sad to see you going thru this BS at the hands of the
commercial antenna makers. I agree with Frank - I've
seen their work and gag  inside every time I look closely
at their element mountings, boom materials and general
structural integrity.  Just imagine that Frank had to double
the weight of his Tennadyne log to make it air worthy. I
was at his place and saw his mods and they were right
on the mark.

Anyway... there is no need for an expensive crane to put
up a Yagi. Run a 45 degree angled "tram-line" rope from
the ground up to the top of the tower. The Yagi slides up
on a pulley pulled by another rope and pulley. Use a
come-along to position/pull it up on the top mast mount.

I also agree with Frank on building a log. I would take
that trap Yagi down, [with a tram line rope] pull it apart
and use the materials to build a nice 13-30mhz log periodic.
Go to the metal scrap yard to fill in the missing aluminum
for CHEAP. Put the new traps on e-Pay for some sucker who
blew his own out.

You could get the Tennadyne 30' boom element spacing
and length details and duplicate it in your heavier duty version.
Bullet proof.

You will learn a lot in the process, which is the best part.

Here's  the 12-33 mhz JJ stacked log periodic system - homebrew
and put up with a tram line rope at 100' and 60'.  Top log is 60'
boom, 400 pounds, overkill - been up for 6 years now. Bottom
log unfinished, good for 20M for now. Notice the "star guying", 6
point guys connected to HB steel triangles to greatly reduce tower
twist from the antennas:
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« Reply #10 on: August 03, 2005, 11:33:00 AM »

my log is 30 feet long boom with 12 elements at 67 feet. Takes power like a dummy load. My log is about 90 LBs.
My heart sunk when I opened the box my antenna came. I felt taken.
I informed tennadyne of my feelings and share them every chance I can.
Electrical performance is very good. I've also had this conversation with the new owner.
I wanted them to make a heavy duty version that would be very cool.

Tom saw my design and built an even stronger one.

Dean KNX and I walked it through the bands one Saturday morning running about 50 watts. Chuck was on the side to strap me with his stacked logs.
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« Reply #11 on: August 03, 2005, 11:39:40 AM »

Quote from: WA1GFZ
My heart sunk when I opened the box my
antenna came. I felt taken. I informed tennadyne of my feelings
and share them every chance I can. Electrical performance is very
good. I've also had this conversation with the new owner.


I seem to remember when you told the Tennadyne guy that his
log wouldn't last long, he said to you something about hams taking
down their antennas every few years anyway, so what's the
difference?  How did that reply go?... :lol:

Well, believe it or not, I'm on my way out to work in this heat to
do some needed overhead and side trussing on the new 6M array....

T
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« Reply #12 on: August 03, 2005, 11:48:18 AM »

YUP, then I told him I just took down my homebrew Quad that was up 19 years.  

The only thing you can do is spread the word to protect your friends.
The hardest thing about homebrew antennas as you know is finding a source of materials. Just think how a little find like Frank in W. Hartford
made all the difference in the world for both of our projects.

G.L. on the project. I'm almost done sheet rocking the ceilings so know about generating sweat.
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« Reply #13 on: August 03, 2005, 02:53:44 PM »

I am not suprised at the response you received from Cushcrap.  I have a Mosley TA 53 and on the first assembly, I ran into problems which prompted a couple of calls to them.  The first call was reasonably congenial, but the second one was as if I was trying to get company secrets.

They act like, we sold it, you fix it.  The only company I have dealt with that was very nice across several conversations was Hustler (Neutronics)  I have one of their 5BTV antennas and spoke about moving the 20 meter section to 17.  They were very nice and made repeated suggestions (it never did work, so don't try).

The Mosley is made well mechanically, fit on the lot and gove me the frequencies/bandwidth I wanted.  I have moved it 4 times and each time it went together without problems.  I like the log periodic thing, but you have reduced front to back gain to get the bandwidth.  However when the bands are in, who cares.

A fellow named Curt Steuba (sp) writes a good column about antenna (called Aerials) in that magazine from Calif. that looks like a tabloid.  I can't remember what it is called, but he lays it on the line.  My kind of ham.
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« Reply #14 on: August 03, 2005, 03:59:21 PM »

Quote from: Jim, W5JO
II like the log periodic thing, but you have reduced front to back gain to get the bandwidth.  


Hi Jim,

This is an interesting topic. Many opinions, of course. Here's
mine...

Generally speaking, you are correct. There's no free lunches. But
there are many variables to consider. This is the case when looking
at the smaller, compact logs and ones that are poorly designed, or
when a Yagi is tuned for max f-b. ...  But, boom for boom length, they
are within a db of gain and the f-b is similar to a Yagi when the Yagi
is tuned for reasonable forward gain. A Yagi tuned for MAX
forward gain has a f-b of maybe 15db, so a log may have a better
f-b in that case. A yagi tuned for max f-b can exceed 45db over a
narrow range, but will lose a ~ db in forward gain, thus back to par
with the log. [of similar boom length]

Logs get a bad rap sometimes. I thought the same thing before I
looked into them closely.  A lot depends upon the design
and implementation. There are a few commercial logs out
there that use openwire line to inter-connect the els. Big
mistake. The eles are ~ 70 ohms and need a good set of 3/8"
spaced 1" angle aluminum or a double boom to simulate a 70
ohm feedline. Without that some els will hog RF and others will
go along for the ride. Axe Chuck/K1KW about his ideas on this.

As for front to back, mine gets close to 25 db on 20M  and
sometimes 30db + on 15 and 10M, and the Warc bands.
There are times when I turned the log and could not even
hear guys who were previously in there with decent signals.
Especially on the bands above the lowest design freq, there's
a lot of "reflector aluminum" behind the driven el...  Cheesy

As for forward gain, yes there is a compromise, and much
depends upon the log's taper and spacing factors. You can
put up a 13-30mhz log with a 20' boom and it wud be lucky
to perform like a 2el Yagi, poor 12db f-b, etc. BUT, put up one
that covers the same freq with a 60'++ boom and you are
seeing performance that equates to 4el Yagi monobanders.  
Chuck/K1KW, who also has a HB stack of 60' logs, says that
they are the equiv of a  4-5el Yagi monoband stack - but
covers 13-30mhz continuous!  But, as you suggested, you can fit a  
wide spaced 6el monoband Yagi on a 60' boom with somewhat
better performance than the log.

Also, consider the fact that logs are virtually unaffected by
tower problems, like guy wires, other beams,  ant wires, etc.
Being a closely spaced, all driven cell, they are not like a Yagi
whose performance can be destroyed by surrounding objects.
I wud venture to guess that many hams with Yagis that are
stacked with other beams and guy wires  "think" are working
well but are not. It takes very little to ruin a f-b on a sharply
tuned Yagi. A log can actually sit 5 ft off the ground and still
work with a pattern. The take off angle will be very high, but
the elements will still have the proper phase relationship,
though pulled down in freq.

BTW, Chuck/KW has been known on the 10-20M bands as
having probably the biggest signal and best ears in North
America. He gets on and the pile ups are bedlam.

As you can see I love the log periodic, and when we consider
that there is no el tuning required, the gain and f-b, AND SWR
stays within a reasonable limit from 13-30mhz, the gain and
f-b rival big monband Yagis [assuming they are big logs], it's
a hard combo to beat.... Cheesy  But takes more effort, more
elements, to build a big set of logs - no free lunch.

BTW, modeling a log with just 180-0-180-0, etc feed will not show the
full gain. Using Nec and a simulated feedline is closer. According
to Chuck, there is a capacitive charging effect on the smaller elements
in front of the normal cell that adds to the gain and will not show up
in some models. I've never verified this, but thought I'd mention it.

73,
Tom, K1JJ
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« Reply #15 on: August 03, 2005, 04:25:43 PM »

There you go getting real techincal on me.  Most of us have a modest budget for tower and rotator.  You are very correct on all you stated Tom but in my case and most people the size to get what you describe would involve a LOT.  

I sure wish I had the support and rotator to support a proper log.  I worked around a 25 KW log one time.   The boom was approaching 100 feet.  What an antenna, and works just as you describe.  But for a 16 ft. boom I see about 13 dbd average on everything but 12 meters for f/b and about 7 dbd forward.  For a 16 ft. boom that works for me. :roll:

The real good point of the LP is the effect of surrounding items as you describe.  As for SWR, as long as it doesn't hurt the feedline or transmitter, I don't worry about it, efficiency is what counts.

If you should come up with another tower, rotator and LP like yours, please deliver and install, I will furnish beer and pizza.  Oklahoma is a short drive for an 18 wheeler. Cheesy
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« Reply #16 on: August 03, 2005, 04:54:27 PM »

OK Guys
This has been educational indeed. I'm sorry I do not have a lattice tower. It was going to be twice the price plus dealing with municipalities and permits and all that didn't interest me. So, I'm stuck with a nice 75 foot utility pole. Any time there's a problem with the Cushcrap I'll have to hire the crane and a hardy soul who will do any necessary maintenance.
Most of the Hams here in town are too old to climb. There are 6 Hams within 5 miles of me. I'm sure they will love the AM carrier. Hopefully they are 2M types.
I'll give you some feedback when I get a connector on the coax and do some serious sweeping with the MFJ.

There was one other question I had about the elements not being perfectly aligned to each other. The Director is off a little. Hopefully no big shakes.
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« Reply #17 on: August 03, 2005, 05:13:27 PM »

Quote from: flintstone mop
There was one other question I had about
the elements not being perfectly aligned to each other. The Director
is off a little. Hopefully no big shakes.



Hi Fred,

No, the slight misalignment [I assume it's like 5 degrees max?]
will have no effect you would ever notice. Their average distance
spacing apart is still the same, so the tuning will be OK. If the el
alignment was off by say, 15 degrees or more, then you might
BEGIN to see coupling problems between els and the pattern wud
start to deteriorate.  

Jim...  Yes, I fully understand the tower/QTH limitations. You do
what you can do with what you have, for sure.  

There's been places where I've read that logs were simply a
bunch of dipoles that had no pattern at all.... just a broadbanded
discone type ant.  :roll:  I just wanted to express the fact for the
BB readers that may not be aware,  that logs indeed have very
strapping patterns and have their place.  In fact, if I were limited to
a single 30' boom antenna, it would indeed be a 12 el log like what
Frank has up. It would be a wee bit more work than a TA-33, but even
a  Rohn 25 tower would handle it.

Thanks for bringing up the point, OM.

T
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« Reply #18 on: August 03, 2005, 06:23:00 PM »

Tom Vu,
You forgot to mention front to side on the log. I have never seen an antenna that matches the front to side. front to back. That is what QRO
dusts away.
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« Reply #19 on: August 03, 2005, 07:17:42 PM »

Fred:  Perfect horizontal alignment should not be necessary, if it is that critcal, something is wrong.  Get the connector on there and get the readings.  I will be curious to know.  

One of the worst antennas I have ever used is the KT 34 by KLM.  Compared to others it had good F/B and gain, but mechanically was a nightmare.  It isolated the elements from the boom and used some sort of plastic attachment points.  Wind is West Texas would move those in no time.

I sure wish I had the room and support structure for a 30 ft. boom.  I would have a LP up in a heartbeat.  Wait a minute, you are talking real money.  You know " a billion here and a billion there and the first thing you know you are talking about real money".
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« Reply #20 on: August 03, 2005, 07:58:43 PM »

I've been running a Wilson System 1, 5 element tri-bander, since 78. The only time I had to take it down was in 91 to repair some wind damage to the top of the tower. At that time I replaced all the beam nuts, bolts, U-clamps, and saddles with stainless steel hardware. Great antenna.
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« Reply #21 on: August 03, 2005, 08:53:57 PM »

Jim,

You bring up a good point about element spin on
booms.  Nothing looks more JS than a Yagi that's sitting up there
with spun eles after a wind storm... :lol:

I've had a few do that in my earlier years...mostly becuz
of high winds and using only u-bolts.

But at this point, all of my elements are double u-bolted AND
pinned to the boom with bolts right thru the el plate and boom.
Even the 6M els are pinned. It's EZ to do and becomes bullet
proof in the highest winds and ice loading.

As far as $$... look up the scrap metal yards in your area. The
one I hit had YEARS of built up used aluminum boom and
element material. $1.50/pound.  It seems no one uses this
stuff for much, so the selection can be FB.

I've built some elaborate antenna arrays for under $200 of
aluminum. [That's 133 pounds of alum stock].  They have
1/4", 3/8" plate, etc for your brackets too.

Bob, K1KBW just built a 4 el 6M Yagi, complete, for $21 of
alum from this place.  

Check out the steel area for bargains on masts, etc. too

Frank, yes, the front to side on a log is good. Though, even a
dipole has a great f-s ratio, right?... Cheesy    I'm curious  what ant
situation you are comparing the log's f-s to?

73,
T
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« Reply #22 on: August 03, 2005, 09:09:48 PM »

Tom:

In Oklahoma the junk places only have old cars and refrigerators.  I probably would have to import the materials Smiley
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« Reply #23 on: August 03, 2005, 09:44:29 PM »

Jim,

Do a search for metal recycling companies and metal dealers with
used metal.  Check the phone listings and also call the local
machine shops. The owners usually know where to find this
stuff used.

If they don't exist in Oklahoma, there's a big opportunity for you... Cheesy  
The guys I know in this area doing it are millionaires as a result.  
BIG business this recycling. The prices on copper, alum and steel
are the highest in years with a big spread to work with.  They hold
the best scrap and put in on racks for retail business too.   Check
around..

T
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« Reply #24 on: August 04, 2005, 10:01:28 AM »

tom
I was turning the antenna while W1AW was transmitting and found the null very deep FS.   I get about 15 dB on 20 M and over 20 dB on 10.

I took the tennadyne square booms and welded sleeves into them.
Them put fiberglass blocks between both booms with holes for the elements. Nothing has moved since '97. It had about 1/2 inch of ice
a couple times.

Yup you can build antennas a lot cheaper than buying them. it is a lot of fun. I was amazed how well it worked 6 feet off the ground. The best place for stainless is mcMaster but the Boston Whaler boat place in Niantic
is also quite cheap.
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