Tennadyne T8 Antenna Project

My Evaluation of the Tennadyne T-8 Log Periodic Antenna

Hello, please take a few minutes to read this write up. It may help you during the assembly phase or give you insight as to the superb material and design of the Tennadyne T-8 Log Periodic.

My journey with the Tennadyne T8 started with an inquiry for a Cubex quad, so I reached out to the company and Mark Chouinard K5YAC responded. Mark explained to me that he just purchased the Tennadyne and Cubex quad companies and was still getting organized after relocating the companies to Oklahoma. After several discussions I initially wanted the T10 HD but due to logistics and delivery time I settled on the T8.

The antenna arrived within a week from Oklahoma and showed up in an approximately 6’ long by 6×6 box. As in typical ham fashion I immediately opened the box to inspect the contents and locate the manual.

That night I thoroughly thumbed through the manual and noted general assembly procedures. The manual was not easy to get through or use when constructing the antenna. The pictures were very poor and lacking in detail.

I had the opportunity to speak with Mark several times through this build and he said that the manuals are on the radar to be updated.

The first head scratcher I encountered was during the boom assembly and splices. It took me a bit to realize that the screws and locknuts used actually go into the boom and not through the boom. If you look at the boom sections and the boom splices you will see that one side has bigger holes. That’s the side the screw goes into.

I have to point out that the hardware is Stainless Steel and the machining of the boom and elements are top notch. The tolerances are very tight and fit perfectly some very slight sanding with fine emery cloth may be needed, but not in my case.

I decided to go with a ferrite CMC (Common Mode Choke) instead of the Collins choke so I did not use the choke plate that came with the antenna. In my opinion this made for a cleaner installation.

Assembling the elements is very straight forward, you will notice that some of the tubing has bigger holes for the screws, just be mindful of this and follow the instructions regarding a machining mark to indicate the tube end that goes into the boom sub assembly and you should be fine.

The only part of this assembly that I question was the placement of the shorting stub on the outside of the boom. This allowed for the possibility of the shorting stub to loosen and pivot from side to side. I decided to place the tuning stop inside of the boom sub assemblies. This allowed to limit side to side travel, unfortunately when trying to tighten the bolt holding the shorting stub it began to compress the tubing in the boom sub-assembly. I drilled additional holes and put bolts through the shorting stub which created a solid electrical and mechanical connection.

 

What a Fiasco....

READ THIS!   I managed to get the antenna assembled and installed with the help of my son. I raised the antenna and heard booming signals on 10-20 meters. I began checking the SWR and tuning and it was horrible, all the bands were 3+ SWR I think 18 meters was almost usable without a tuner. I checked everything and reviewed the manual, in final desperation I reached out to Mark Chouinard on Christmas day and he actually responded.  I sent him several pictures and he forwarded them to Roger who used to own Tennedyne and Cubex. Mark reached out to me on Christmas day and asked me if I was trying to build a yagi. Needless to say I immediately had that sinking feeling and after the dust settled I realized that I screwed up the element to boom placement. After researching this I realized my mistake and the elements needed to be 180° out of phase to work properly. Sooo this meant lowering the antenna, taking it off the tower and splitting the boom again.

WRONG

Correct

Performance , Problems, Opinions and Solutions

Once I fixed the problems I caused,  the antenna worked as described on all bands but 12 meters. The SWR is 4+ on 12 meters and I did a search regarding this. I found that running the coax along the bottom boom which is hot was causing CMC (Common Mode Current) even with a choke at the feed point.

I found this article:

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

My experience with logs is limited to some broadband commercial TV band arrays used in MATV and CATV head ends. I worked as an independent consultant for two companies that manufactured heavy duty multiple channel antennas for commercial installations. In these installations, pattern and SWR was everything. Because the antennas were used to feed video to thousands of customers, and often installed hundreds of feet above ground, the antennas needed clean tight patterns without ghosts caused by standing waves.

I actually became involved because some commercial antennas had ghosting and pattern problems. These problems were related to improper feed systems. This is a problem that appears not only in commercial TV antenna designs, amateur antenna manufacturers also place baluns at the incorrect location in LPDA arrays or instruct users to route the coaxial feed cable incorrectly. This can cause substantial RFI and all sorts of unusual problems, such as excessive RF into house wiring or consumer electronics. Improper cable routing can also cause odd SWR problems. If we consider what the balun does, we can see how the mistake occurs.

I was first made aware of an amateur radio log periodic feed problem by a friend who had severe consumer RFI problems whenever he ran an amplifier. The following describes what we found.

What’s Right or Wrong with LPDA Feeds

Cold Booms

Some antenna manufacturers suggest taping or attaching the coaxial feed line along the length of the boom, back to the mast. This would work perfectly in any antenna with a “cold” boom. The coax shield and the boom would be at the same potential, and the boom would also be grounded directly to the mast and tower, placing the coax shield, boom, mast and tower all at the same RF potential, hopefully zero or near zero RF potential. These log designs almost always have a high impedance transmission line, generally 200-300 ohms. 200 ohm systems are commonly used in transmitting arrays, through a 4:1 balun, to a traditional 50-ohm feed cable. For receiving, balanced line impedance is increased and the log optimized for a 300 ohm feedpoint, allowing a 4:1 balun to convert to 75 ohm cable. As a general rule though, logs work better with lower impedance transmission lines.

Hot Booms

Some log periodic designs use the boom as one conductor of a two-conductor balanced transmission line. This is a very good thing to do, if the system is managed properly, because it is possible to build a very low impedance balanced transmission line. Lower impedance balanced lines in the log tend to produce better overall log performance. The Tennadyne log periodic is one example of a “hot boom” antenna.

In hot boom antennas, two booms run parallel over the length of the array. The booms are insulated, or should be insulated, from the mast and tower. Elements alternate connection to the upper and lower, or left and right, booms. This inverts phase, as transposing adjacent element connections in cold boom antennas does.

 

balanced transmission line log periodic

 

A balanced transmission line, such as the balanced line feeding elements in a log periodic, must have the following:

Equal and opposite voltages from 1 and 2 to point A, and from 1 and 2 to point C, at each end

Equal and opposite currents in 1 and 2 at any point along the line

Equal and opposite voltages at point B all along the line

 

 

 

 

Unfortunately, some instructions route the coaxial feed line directly along the hot boom. This is exactly like taping coax to one conductor of a balanced open wire transmission line, because the boom is part of a two-wire transmission line! Both booms are electrically hot, just like conductors in any balanced transmission line. Each boom is, after all, feeding half the elements!! Obviously the boom cannot feed elements properly without equal and opposite voltages, and equal and opposite currents, along each boom.

Coax taped to boom

 

When coax is taped to the balanced boom, either balanced conductor line 2 is forced to zero, or the coax shield is forced above ground. Actually a portion of both happens, with the coax shield becoming a radiator and boom conductor 2 becoming more at the potential of point B.

 

This forces the coax to radiate and create RF ground loops, while also starving half the log elements of voltage and current.   

Any additional conductor or feed cable paralleling the boom, especially one held directly against the boom by tape or wire ties, will be significantly excited by boom currents and voltages. Cables routed against or near the hot boom (both booms are hot when the booms are used as transmission lines) will have the voltage difference between the boom and the tower at the cable exit point exciting unwanted external currents in the feed line shield. The feed line shield will also be excited by magnetic coupling by magnetic fields created by boom currents. In effect, the coaxial transmission line shield becomes a third transmission line conductor, driven by boom currents and voltages through the very tight mutual coupling.

The coaxial feed line shield is supposed to be at ground potential. It must attach to the antenna in a way that keeps the shield at “ground potential”.  Otherwise, the shield will radiate and also bring significant RF voltage away from the antenna. At some point we must have a properly thought-out balanced to unbalanced transition.

As with any balanced transmission line, the hot booms (or a balanced two-conductor line running along a cold boom) should either be symmetrically spaced a reasonable distance in relation to the two conductors forming the balanced line, or placed several conductor spacings away from the hot conductors that form a two-wire balanced line. Ideally an external feed line exiting the antenna would not be within a few feet of large diameter hot booms.

A second method that only works with a 50 ohm feed (this will not work with 200 or 300 ohm feeds) would be to tape the coaxial feed line along the shield attachment point boom and treated like a part of that boom.

 

 

LPDA feedline boom routting TVI RFI SWR problems

 

In this example, the coaxial cable shield is much closer to the hot lower-boom than the hot upper-boom.

We would never dream of lashing a cable, or any external conductor, to one conductor in a two-wire balanced feed line feeding a dipole.

Commonly available information instructs us to keep all external conductors several balanced-line-widths away from any unshielded balanced line.

 

 

 

 

 

 

There are two ways to correct this problem:

Method 1.

This is a universal method for any balun style.

correct log periodic LPDA feedline routing

 

 

This is the electrically ideal method of bringing the feed line back to the mast from the log feed. This method keeps the coaxial line away from the balanced transmission line formed by the boom. This works with ANY log periodic and good balun, including 200-300 ohm balanced systems.

 

 

The sole drawback with this method is mechanical. The coaxial feed line often requires additional mechanical support, such as a non-metallic messenger rope or an insulated rod.

 

 

 

 

 

If the log has a 1:1 balun at the feedpoint AND if you use a current balun, you can simply tape the coax to the boom that the shield connects to without any balun at the log feedpoint. Ground the coax shield to the hot boom at the exit point where the coax leaves the boom, and install a balun at that point. This can be done by installing a barrel bulkhead feedthrough connector at the exit point and clamping the connector to the boom. Be sure to waterproof the assembly. The new balun goes between that boom connected barrel connector and the feed line exiting the antenna boom and running down the tower.

 

 

 

Method 2

This method only works with current baluns. While mechanically better, it is limited electrically to current baluns, or no balun, at the log feedpoint. If the feedpoint balun is a voltage balun, either use method 1 or change to a current balun.

 

 

proper LPDA 1:1 impedance feed system log periodic

  

If the beam has an impedance-matching current balun at the feedpoint, in other words a balun with an impedance ratio other than 1:1, the original balun must be left at the feedpoint to provide impedance matching.

If the balun is a current balun, and cannot easily be reached, it is perfectly acceptable to leave the original balun at the feedpoint. Leaving the balun at the feedpoint won’t bother anything so long as the balun is a current balun.

In this case, the coaxial cable is cut and a good common mode isolation device is installed where the coaxial cable exits the boom. The isolation device can either be a 1:1 current balun, a common mode choke, or a “feed line isolator”. All are electrical equivalents.

 

 

At the feedpoint, the coax shield must be connected to the boom that connects to the shield. If the coax routes through the inside of the boom, the coax shield must be connected to the boom coax with the coax routed inside.

In the hot-boom TV-channel log feed systems I designed, the unbalanced coaxial feed line ran through the shield-connection boom center. The feed line exited the boom via a feedthrough connector at a zero voltage point along the boom.  This eliminated need for any balun, and greatly improved lightning protection.

 

 

Conclusion

For proper operation the feed line must be suspended at least a few feet below the hot booms all the way from the balun’s unbalanced connector to the tower or mast (this works with any balun and any impedance log), or a second 1:1 current or choke balun must be added at the mast (this works most effectively or reliably when a CURRENT balun is used at the feedpoint, or when the feedpoint requires a 1:1 balun and no feedpoint balun is used). This will greatly reduce RFI and improve the antenna pattern and gain.   

Performance and Conclusions

So here is the meat and potatoes of this antenna. I am glad I purchased this antenna, this design is a performer no doubt I have not needed my amp AT ALL for any of my 10-20 meter contacts, I enjoy DX and have made contacts globally with 100 watts usually on the first call with surprised responses when the DX stations finds out I am only running 100 watts.

You will not have a 1.0 swr, but you will have a good SWR, all the bands are showing 1.2 to 1.5, this is more than acceptable. The antenna looks very cool and is light weight enough for smaller rotors like the ham 4. It works really well and the specs seem to be in line with what I am seeing. I would definitely recommend this product and the customer support from Mark was superb, even above and beyond responding on Christmas day.

We had an out of the norm snow storm in Virginia that dumped almost 24 inches of a heavy wet snow in less than 24 hours. The snow damaged my GAP titan and the Tennedyne T8  came through it perfectly.

I did want to test the T8 with the booms covered in snow but my power was out for almost 5 days, I wanted to see how the snow would react while it was shorting the booms.

I rate this antenna 4.8 stars. I would have given it a 5 star rating were it not for the lacking manual and the 12 meter issue due to the CMC caused by running the coax along the bottom boom.

Tips and Mods

While speaking to the guys on 80 meters some of them suggested putting dowel rods or fiberglass rods into the element where it enters the boom for support. The old timers said in the past that was a failure point on the Tennedyne antennas.  I can see that happening and I would suggest welding a solid aluminum rod into the boom section that you would put the element onto. This would make it very sturdy and fool proof when guys like me try to build a yagi out of a log periodic antenna.

The other tip was to get the coax at least 24” off the bottom boom and for a clean look use PVC and build yourself some stand off’s. I have some that I built and once they are attached I will include some pictures.