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Author Topic: TENNADYNE T-8 LOG PERIODIC PROBLEMS  (Read 6213 times)
W8KMX
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« on: July 30, 2011, 01:14:17 PM »

I have a friend who has a T-8 that does not work very well. He has has it several years and says that it never worked very well. I have read several reviewes and all say it is a great antenna so I am sure it is capable - I just have to figure out what is wrong and what needs to be done.

Basically, the SWR and Z is very high everywhere except 20 meters. It is just not usable on any other band. I saw one review by KB5OK that said he installed part of the beam backwards and that the manual was not a lot of help. So, I want to know how could I screw up and re-assemble it backwards etc?

I also need to get a "good" manual. Any suggestions would be greatly appreciated.

Reply here or direct to w8kmx@bellsouth.net.

Regards,

Dan
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K3SF
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« Reply #1 on: July 30, 2011, 05:33:22 PM »

hi

i have a T-8 and has worked just fine for me

on yahoo group
http://groups.yahoo.com/group/TennadyneAntennas/
there are many there with T8s and a lotta  good advice

i also photographed my complete assembly of the T8 which can be found at
http://groups.yahoo.com/group/TennadyneAntennas/photos/album/1719437328/pic/list

from unboxing to raising..

manual doesnt offer much except step by step instructions which can easily get confusing
which is why i took pictures to help double check me as i went thru the process

no final measurement..

use of a balun is important..also more good advice from the group...


hope this helps some

Paul K3sf


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W8JI
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« Reply #2 on: July 31, 2011, 10:35:37 AM »

Tennadyne, for some weird reason, has a very poor understanding of baluns and feedline routing. They seem to refuse to learn or understand how a balun works and how to route the feeder.

They tell people to put a balun at the feedpoint, then route the feedline back along the HOT boom of the antenna. That's just a disaster waiting to happen.

http://www.w8ji.com/baluns_on_log_perodic_antennas.htm

Another company heading for problems on baluns is Force 12. I looked at a "balun" revision they had that was supposed to come from some Ham in England and it is not a balun at all!!!!


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AB5Q
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« Reply #3 on: July 31, 2011, 08:49:19 PM »

I've run the typical Collins style balun at the feed point on my logs for well over a decade and have never seen the problem that Dan describes. So I highly doubt that his problem is related to the position of the balun rather something more fundamental.  Dan, I sent you a note with some questions and photos, I am quit familiar with the T-8. Please let me know if the email doesn't make it through.  We'll help you figure this out.

73,
John
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W8JI
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« Reply #4 on: August 01, 2011, 07:46:00 AM »

I've run the typical Collins style balun at the feed point on my logs for well over a decade and have never seen the problem that Dan describes. So I highly doubt that his problem is related to the position of the balun rather something more fundamental.  Dan, I sent you a note with some questions and photos, I am quit familiar with the T-8. Please let me know if the email doesn't make it through.  We'll help you figure this out.

73,
John

Here is the hard cold fact that is irrefutable:

With a hot boom on the log and a balun only at the feedpoint, the coax cannot be routed back along the boom without ruining balance in the system.

If the coax is routed back along the boom the shield needs bonded to the shield side hot boom at the exit point and a balun installed there, or the balun at the feedpoint method demands that the coax be kept well away from the booms.


If the coax is routed along or near the boom, there will be problems with balance if the balun is only at the feedpoint. That's a fact.

73 Tom
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AB5Q
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« Reply #5 on: August 03, 2011, 09:34:13 PM »

W8JI - (Tom) I don't think that anyone refutes your balun placement theory at all, although it would be nice to see some hard cold data to better understand the magnitude of the problem and to quantify the anticipated performance benefit.  I agree that there is a certain degree of coupling between the transmission line (outer portion of the shield) and phasing line (lower boom). However without going into too much theoretical detail it is in my opinion that the amount of coupling would be minimal.  There are other environmental factors that will have a more adverse affect on an antenna's radiation pattern than the proper or improper positioning of a balun.

I’d like to reiterate another hard cold fact that I have used Collins style “chokes” at the feed point for well over a decade on a variety of LPDA antenna configurations and installations from 13-1300 MHz with boom style phasing arrangements. The antennas have always performed well beyond expectations with reasonable SWR (<2.0:1), clean patterns and deep FB and FS nulls.
 
On a final note regarding your proposed balun arrangement, I would encourage you to reevaluate the need for connecting the transmission line's shield to any part of the phasing line (lower boom) other than at the front of the antenna. I ran a simulation and the redundant shield connection midpoint on one of the phasing lines is very detrimental to the antenna's radiation pattern. I understand what you are trying to address however you would be better served not making the secondary connection and simply choking the coax at the mast and leaving it at that.

73,
John
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W8JI
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« Reply #6 on: August 04, 2011, 05:48:15 AM »

W8JI - (Tom) I don't think that anyone refutes your balun placement theory at all, although it would be nice to see some hard cold data to better understand the magnitude of the problem and to quantify the anticipated performance benefit.  I agree that there is a certain degree of coupling between the transmission line (outer portion of the shield) and phasing line (lower boom). However without going into too much theoretical detail it is in my opinion that the amount of coupling would be minimal.  There are other environmental factors that will have a more adverse affect on an antenna's radiation pattern than the proper or improper positioning of a balun.

Why you may *think* running the coax along a hot boom will not cause substantial coupling, such a poor practice actually is a worse case situation. It actually is often worse than not using a balun at all!!

Think of it in the context of a dipole. Would you install a balun at the center, and then tape the feedline along one leg of a dipole to exit at a convenient point? Why then would anyone with a smattering of understanding of antenna run the coax along a boom that carries all the current out to half of the antenna elements?

You might wonder how I got involved with this. A friend of mine bought a Tennadyne log and had terrible RFI and TVI as well as odd SWR behavior, even though he followed the instructions to the letter. We simply tilted the antenna so we could access the coax, cut it free from the boom, suspended the cable through air a few feet below the boom, and all of his problems vanished. No other changes.

Now if he was lucky he might have accidentally picked a cable length and tower height (his cable was taped to the tower) that nulled common mode, but we elected to fix the actual problem and not go with manufactured luck.

By the way, years ago I designed some commercial TV band logs. They were much like the Tennadyne with two hot booms. I brought the coax through the lower boom out the very back end (where there was a shorted reflector element) at a zero voltage point of the boom and exited there through a connector. The coax ALWAYS has to leave the area of the antenna at a zero voltage point. 
Quote
I’d like to reiterate another hard cold fact that I have used Collins style “chokes” at the feed point for well over a decade on a variety of LPDA antenna configurations and installations from 13-1300 MHz with boom style phasing arrangements. The antennas have always performed well beyond expectations with reasonable SWR (<2.0:1), clean patterns and deep FB and FS nulls.
 

It does not matter what type of balun you use. If you route the feedline back along a hot boom you are "undoing" the balun.

Quote
On a final note regarding your proposed balun arrangement, I would encourage you to reevaluate the need for connecting the transmission line's shield to any part of the phasing line (lower boom) other than at the front of the antenna. I ran a simulation and the redundant shield connection midpoint on one of the phasing lines is very detrimental to the antenna's radiation pattern. I understand what you are trying to address however you would be better served not making the secondary connection and simply choking the coax at the mast and leaving it at that.


That makes no electrical sense at all. Most likely your model is wrong.

If the coax shield is bonded to the boom it simply becomes part of the boom. The only point a balun is needed is at the exit point from the boom. This is just the way conductors and transmission line work, and I am positive of it. Anyone understanding transmission lines and feed systems would agree.

73 Tom
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N3JBH
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« Reply #7 on: August 04, 2011, 06:35:05 AM »

If your bonding the shield of the coax to the boom which i understand and makes good sense. I wonder why you need two baluns one at feed point and the second on the mast? Wouldn't the one at the mast be all you really need? I don't see what the other one at the feed point would really solve in this case.    Jeff
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AB5Q
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« Reply #8 on: August 05, 2011, 12:47:00 AM »

Why you may *think* running the coax along a hot boom will not cause substantial coupling, such a poor practice actually is a worse case situation. It actually is often worse than not using a balun at all!!

Think of it in the context of a dipole. Would you install a balun at the center, and then tape the feed line along one leg of a dipole to exit at a convenient point? Why then would anyone with a smattering of understanding of antenna run the coax along a boom that carries all the current out to half of the antenna elements?

Poor analogy, you are suggesting that the boom (balanced low impedance phasing line) and antenna’s associated elements are both equivalent radiating bodies. This suggests that parallel conductors carrying equal and opposite currents will radiate just as effectively as conductors that are at 180 degrees of separation. The theory that radiating electric fields preferentially seek and transfer appreciable amounts of energy to a non-resonant body seems a bit presumptuous.  Your assumptions contradict basic transmission line theory as well as some of the fundamental principles behind the LPDA design. I don’t suggest that coupling never occurs between the lower boom and the unbalanced transmission line rather the coupling is minimal at most. I do suggest that feed line radiation is the direct result of imbalances in the antenna’s radiating elements where the electric field is at its highest potential.  The imbalances are caused by obstructions within the near field of the antenna more than through coupling of the transmission line with the boom. The Collins balun is far from ideal but its simple and a reasonable compromise.  The current balancing efficiency of a Collins balun is marginal when compared to a properly designed ferrite balun.  So integrating a properly designed current balun at the feed point of the antenna in my opinion would minimize most issues experienced with erratic LPDA performance.

Regarding your proposed balun arrangement, I would encourage you to reevaluate the need for connecting the transmission line's shield to any part of the phasing line (lower boom) other than at the front of the antenna. I ran a simulation and the redundant shield connection midpoint on one of the phasing lines is very detrimental to the antenna's radiation pattern. I understand what you are trying to address however you would be better served not making the secondary connection and simply choking the coax at the mast and leaving it at that.
That makes no electrical sense at all. Most likely your model is wrong.

There are possible errors, nothing is perfect however for the most part the result would be the same. I’m sure that you are familiar with a simple two-element broadside ½-wave collinear array where the parallel elements are fed 180 out of phase (sound familiar?). Let’s say hypothetically that the center portion of the balanced phasing line is fed with an unbalanced transmission line and a simple choke, all impedances are normalized. Would you expect currents between the two radiating elements to remain balanced if the shield portion of the unbalanced transmission line where placed a significant distance away from the opposing conductor’s termination point? This would effectively induce an imbalance into a balanced phasing line which would result in the distortion of the radiation pattern of the antenna. This is what you are proposing to do with the balanced phasing system of the LPDA. If randomly placing the unbalanced transmission line’s feed point isn't such a big deal, why not relocate move the feed point of the LPDA to a position next to the mast? I think that you know the answer.

73,
John
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W8JI
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« Reply #9 on: August 05, 2011, 08:57:16 AM »

Poor analogy, you are suggesting that the boom (balanced low impedance phasing line) and antenna’s associated elements are both equivalent radiating bodies.

It is a very good analogy.

In the nearfield, they indeed are virtually the same.

Quote
This suggests that parallel conductors carrying equal and opposite currents will radiate just as effectively as conductors that are at 180 degrees of separation.


It is a distance problem. Most people know that, or should know that. This is why balanced lines should be several line widths away from external conductors. That is where the fields cancel.
Quote
The theory that radiating electric fields preferentially seek and transfer appreciable amounts of energy to a non-resonant body seems a bit presumptuous.



You are the one presumptuous. You seem to think the coax shield is invisible, or has infinite impedance, and has no mutual coupling or nearfield coupling to another conductor!!!

I guess that means every directional coupler and transmission line in the world has to stop working! :-)


 
Quote
Your assumptions contradict basic transmission line theory as well as some of the fundamental principles behind the LPDA design.



Not at all. Only someone who has NO understanding of LPDA's or transmission lines, as well as basic electric and magnetic field behavior, would think taping a piece of coax along one line of a balanced line would be acceptable.

Anyone understanding transmission lines and how they they work would agree with me.


Taping an unbalanced line along one conductor of a balanced line and pretending it is OK is the pinnacle of poor engineering.
I can't even imagine why anyone understanding antennas would ever think it is OK.

http://www.w8ji.com/baluns_on_log_perodic_antennas.htm



73 Tom
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W8JI
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« Reply #10 on: August 05, 2011, 09:03:37 AM »

If your bonding the shield of the coax to the boom which i understand and makes good sense. I wonder why you need two baluns one at feed point and the second on the mast? Wouldn't the one at the mast be all you really need? I don't see what the other one at the feed point would really solve in this case.    Jeff

Jeff,

I think my web page explains that.     http://www.w8ji.com/baluns_on_log_perodic_antennas.htm

If not, let me know and I can correct it. The balun at the feedpoint would:

1.) Need to be a current balun. It cannot be a voltage balun.

2.) Only need to be there if it was a 4:1 or some other ratio other than 1:1. If it is a 1:1, and if the feedline is routed through or along the hot boom, then the balun at the feed is not necessary at all.

I'm at a loss why a few people have problems with this. I saw another commercial antenna at a hamfest that changed designs, and now the balun is no longer a balun at all!!! It will now be another antenna that has unreliable or inconsistent field performance, with performance dependent on common mode impedance of the feedline.

73 Tom

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AB5Q
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Posts: 202




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« Reply #11 on: August 06, 2011, 01:40:33 AM »

Tom, there's really no point in trying to carry on this discussion with you. You continue to spin my comments with nothing to add other than mockery and personal jabs.
 
As far as whether or not attaching the coax cable to the lower boom is an acceptable practice, in my opinion it is.  It’s not a matter of a good or bad engineering practice rather it should be considered as a compromise with few alternatives and an acceptably low level of risk. The uncontrollable environmental factors surrounding the antenna are by far more devastating to the antenna’s performance. Good engineering doesn’t require perfection rather an understanding and the effective prioritization of the many compromises surrounding a particular system.

For anyone that is seriously considering attaching the shield of the unbalanced transmission line (coax) to ½ of the LPDA’s balanced phasing line at a location other than the front of the antenna don't even consider it.  It is only a theory without any supporting evidence to prove its effectiveness. Even if data all of a sudden appears, question it, the practice makes no sense from many angles.  In order to better understand the negative affects of such a modification, I decided to model it while performing a few basic calculations to verify my model’s accuracy.  Based on the simulations, the results were quite predictable in that the modification has an adverse affect on the antenna’s radiation pattern. The results show a reduction in forward gain by -2 dBi, a substantial broadening of the -3dB beam width and an offset of the forward and reverse lobes from 5-45 degrees rising with frequency. So when the antenna is pointed to an azimuth of 0 degrees the forward lobe is actually directed towards an azimuth of 5-45 degrees. The broader beam width will mask the problem to some degree where the antenna will appear to be performing as normal.

There are a number of quality publications that cover the information discussed throughout this particular post.  Regardless of what other self appointed “guru’s” think, I would highly recommend a book entitled “Reflections” by M. Walter Maxwell W2DU.  This is really a good read guaranteed to open new avenues for learning about wave propagation, transmission lines and many more useful topics. 

73,
John - SK
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W8JI
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« Reply #12 on: August 06, 2011, 03:34:10 AM »

Tom, there's really no point in trying to carry on this discussion with you. You continue to spin my comments with nothing to add other than mockery and personal jabs.

I appolgize if it comes off that way, that isn't my intention. It is the idea or theory that directly running a cable along a hot boom is acceptable I find so unfathomable.


Quote
As far as whether or not attaching the coax cable to the lower boom is an acceptable practice, in my opinion it is.  It’s not a matter of a good or bad engineering practice rather it should be considered as a compromise with few alternatives and an acceptably low level of risk.


The risk is horrible, because an independent random impedance conductor (the outside of the shield) is placed within a small fraction of an inch of one conductor of a two conductor balanced transmission line. That can never be reliable or good.



Quote
The uncontrollable environmental factors surrounding the antenna are by far more devastating to the antenna’s performance.


It is unlikely conductors large fractions of a wavelength away could affect the system more than something thousandths of an inch away that are taped directly to one conductor of a two-conductor balanced line. If you are going to do that, it would be better to make it part of the line and isolate common mode at the point where it leaves, so the coax and the boom at least behave as the same conductor in common mode.

Quote
Good engineering doesn’t require perfection rather an understanding and the effective prioritization of the many compromises surrounding a particular system.


That's 100% true. Especially with antennas, designers should understand transmission lines and coupling between conductors.

Quote
For anyone that is seriously considering attaching the shield of the unbalanced transmission line (coax) to ½ of the LPDA’s balanced phasing line at a location other than the front of the antenna don't even consider it.  It is only a theory without any supporting evidence to prove its effectiveness.


It's a perfectly acceptable method if the system uses a current balun at the feedpoint.  If the system uses a voltage balun it will not work. As a matter of fact if the system requires a 1:1 balun, then there is no need to use a balun. All that is required is a high common mode impedance where the cable leaves the boom, which can really be anywhere, as well as keeping the shield at the same potential as the boom all along the boom by bonding the two at least at each end. Of course this only works when the cable is a small fraction of boom diameter so it does not upset effective boom diameter and make the transmission line formed by the booms unbalanced.

The UNsound way to route the cable is by simply using a balun at the log feedpoint and taping or tying the coax to the current carrying boom.

73 Tom



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AB5Q
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« Reply #13 on: August 06, 2011, 10:34:11 AM »

Apology accepted. Tone is difficult to assess electronically implied or otherwise.

Quote
W8JI - The risk is horrible, because an independent random impedance conductor (the outside of the shield) is placed within a small fraction of an inch of one conductor of a two conductor balanced transmission line. That can never be reliable or good.

It is unlikely conductors large fractions of a wavelength away could affect the system more than something thousandths of an inch away that are taped directly to one conductor of a two-conductor balanced line. If you are going to do that, it would be better to make it part of the line and isolate common mode at the point where it leaves, so the coax and the boom at least behave as the same conductor in common mode.
 

Sure it’s perfectly understandable, proportionately the electric field emitting from the radiating elements is many more orders of magnitude greater than the electric field emitting from the boom and it will therefore have a greater affect on the overall balance of the antenna.

To further support my position, I modeled a random impedance conductor a few millimeters parallel to the lower the boom that then transitions at the mast clamp to a vertical element pointing downward towards ground level.  This simulates the outer shield of the transmission line with a high impedance choke (current balun) at the feed point of the antenna.  As expected there was no affect to the antenna’s radiation pattern or impedance curve. Why?  Because the parallel conductor ( boom) phasing line, with equal and opposite currents are balanced, and therefore radiation will not occur.  Sure in the real world there's a small amount of radiation however it is insignificant when compared to all of the other fields present throughout the antenna.  Can you guess what happens when I connect the random impedance conductor (coax shield) to a random point along a balanced phasing line (boom)? A phase shift is induced and the antenna pattern is adversely affected as hypothesized and supported through modeling.

Under normal conditions running the coax along the balanced phasing line is an acceptable compromise. It is in my opinion, that as a minimum a good high impedance choke (current balun) needs to be placed in front of the feed point of the antenna away from the balanced phasing lines. This practice is also supported by at least two of the most popular dual boom LPDA antenna manufacturers and my data shows that they are correct in their recommendations.  Furthermore, it is again in my opinion that a choke at the mast is also acceptable as a primary or secondary measure as it would further increase the common mode impedance of the outer shield of the transmission line.

I have high level of respect for your ideas Tom and believe that you have and will continue to bring a lot to the hobby for many years to come.  I look forward to future discussions. 

73,
John
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W8JI
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« Reply #14 on: August 07, 2011, 11:21:13 AM »

Well John, we will have to agree to disagree.

I think it is a terrible idea to lash a coaxial cable directly to one conductor of a two conductor balanced line. I know from past experience it causes major problems in the field. A friend of mine had terrible RFI and TVI, and SWR problems, with his tennadyne log until he moved the cable. I have similar problem on a test range with a TV receiving log and 75 ohm cable.

73 Tom
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