Detuning a Tower from a Vertically-polarized Antenna

In my previous article on a 2-element loop array for 40 meters I issued a caution regarding the potential harm from a tower that is resonant, or just near-resonant, on the band of interest. This type of interaction can destroy the performance of a directive low-bands antenna that is vertically-polarized. Although even a single-element vertically-polarized antenna will excite a tower resonance the impact is typically not a major concern other than the effect on impedance matching. I have a particular concern with tower resonance since that may be only way I can achieve higher performance with a low-height antenna for 40 meters.

As W8JI describes it is possible to detune the tower so that, at least on one band, the tower can be made to effectively disappear. That is, become non-resonant on the band of interest. This allows the vertically-polarized low-bands antenna to meet its potential.

Of course the tower (plus ground and loading due to yagis mounted above the tower) might not be resonant on the target band and therefore there is no cause for concern. But you can only know for certain by exciting the tower with a nearby vertically-polarized antenna for that band. It would be a shame to go to all the effort of designing and building a high-performance antenna that isn't going to work out.

A better strategy is to remove the tower resonance once it is found, and ensure the antenna fulfills its potential. This is not only a one-time concern since, after all, the tower's resonant frequency will not stay fixed for all time: changes to other antennas on or near the tower will shift the resonant frequency, and we all add and remove antennas on an often yearly basis.

Since the weather is continuing to stay cold, windy and generally miserable here in Ottawa I decided to do a little more computer modelling to test methods for detuning the tower. It would be good to know this since if I erect a larger tower this year I will want to build a directive antenna for 40 meters, and due to its inevitable low height I prefer to go with a loop array rather than a yagi.

W8JI provides some general guidelines for designing and tuning what is effectively a trap on the tower. What is missing are specifics. This is understandable since there are many variables that are installation specific. However I don't want to just wing it. This is where EZNEC comes in handy, allowing us to parameterize the design so that we can succeed faster when we spring into implementation.

For the following discussion I will stick with 40 meters and the switchable 2-element narrow diamond loop array from the previous article. The lessons learned should be applicable to other bands and antenna configurations.

The basics of the trap design model are shown at right. Tom suggests a trap length of no more than 3/16-wavelength so I made my trap 5 meters long (A, wires 10 & 14), 0.5 meters wide (B, wires 12 & 13), and centred on an 18-meters tall tower (wires 9-11, with 10 as part of the trap). The 3 wires are ¼" aluminum rod, which make the trap rigid and adjustable. Heavy-gauge copper wire can be substituted for wire 14. The 18 meters tower height was selected since it is the worst case for 40 meters resonance that I previously discovered.

Currents are shown on the EZNEC plot for the case of trap resonance. Notice that the while the current in the trap is high the current in the tower segments above and below the trap are low (they decline to 0 at each tower end). In this view you cannot see the currents on the elements since EZNEC plots those at a right angle to those shown here.

A series capacitor is positioned at the lower-right corner of the trap, where it is most accessible for tuning. I assumed that the capacitor is a fixed, transmitting "door knob" capacitor or an air variable that is both low loss (small equivalent series resistance, which I inserted into the model) and can withstand the voltages present with high power. The capacitor should be protected from the weather with a cover or enclosure, and protected from mechanical strain by, for example, placing it in parallel with an antenna wire insulator.

Safety note: Place the trap high enough that it is out of arm's reach from the ground since there can be high voltages present on the trap when transmitting high power on the test antenna for which the trap is tuned.

The model was developed without doing any calculations. I simply made an educated guess at the inductor dimensions (length and width) and then adjusted the capacitor value (using an RLC load in EZNEC) until the current was maximum at the initial test frequency. This was easier than determining the trap's resonant frequency, even though in practice the latter is usually easier to measure -- I have a dip meter but not a suitable RF ammeter. Either ought to work since maximum current in the trap should coincide with the resonant frequency.

It only took about two minutes of value substitution to find the capacitor value for the test frequency: 49 pf. That's a useful value since I have several 50 pf transmitting ceramic door knobs in my junk box.

However this, as it turns out, is inadequate. I chose a test frequency of 7.02 MHz since that is the frequency at which the antenna gain is maximum. When I checked across the band I found that the F/B and higher-frequency performance were degraded. The F/B is the most valuable metric since any extraneous current will disturb the fine balance of current phase and amplitude between the antenna elements: a F/B of -20 db requires a power subtraction of 99%. Gain is less sensitive to minor phase and amplitude deviations.

Following further experimentation I found that it is possible to adjust array performance by changing the resonant frequency of the trap:

• Frequency of maximum gain (7.02 MHz, 49 pf): The result is as described above. The gain went up by a small amount, about 0.1 db, which is negligible.
• Frequency of maximum F/B (7.08 MHz, 45 pf): This gave the closest match to the gain and F/B curves for the model that has no tower, as was done in the previous article on this antenna. There is some degradation of gain and F/B at the top end of the frequency range (7.2 MHz).
• Frequency higher than maximum F/B (7.14 MHz, 35 pf): Maximum gain dropped -0.1 db (which is negligible) and the frequency of maximum F/B rose to almost 7.1 MHz. Gain and F/B improved a small amount at 7.2 MHz.
• 7.2 MHz (~30 pf): Maximum gain dropped a more significant -0.3 db and the frequency of maximum F/B rose a bit further than the preceding case. Gain and F/B made further improvements at 7.2 MHz, though not by much.

From this I conclude that precise tuning of the trap is not necessary. If you do want to eke out every decibel per my original antenna design objectives the trap should be tuned for resonance between 7.05 and 7.1 MHz. Matching is not a concern since the SWR curve shifts a managable amount as the trap is tuned across the band.

Tuning the Trap

Although this is a purely software model, one I have yet to build and use, the design must be amenable to tuning. Since it is difficult to directly measure the degree of interaction between the tower and the antenna it is best to focus efforts on the trap. Luckily this should work well, as W8JI said and as my modelling seems to demonstrate.

NOTE: If you see evidence of tower interaction during the initial setup and tuning of the loop array you must put that aside until the tower trap is installed and properly tuned. You should only continue tuning the antenna (per the procedure in the loop array article) when the trap is tuned.

Construction and configuration of the trap are the foundation of the tuning system. The horizontal arms (B) are modelled as solid aluminum rods not only for strength but to allow the inductance to be varied. I kept the arms short enough that the entire trap can be reached from the tower.

The vertical arm, A (parallel to the tower), can be wire, just take care that the copper to aluminum junction is solid and protected from corrosion. Solder lugs are a good choice, much better than clamping the wire directly to the aluminum.

The capacitor is placed at the bottom of the trap for a reason: to allow adjustment with the minimum possibility of coupling between the trap inductor and your body. It should be solidly attached to the bottom arm so that it is robust against abuse and tuning (if it is a variable capacitor). Tune it with your head below the level of the bottom arm and good insulation between your hand and the body of the variable capacitor.

If you choose to use a dip meter to tune the trap a small pickup loop should be inserted at the bottom of A. This is easier to do when vertical arm A is wire.

C vs. L -- The trap can be tuned by varying either the C or L component, although until now I've only discussed C. The L value, although not directly measured and difficult to measure in practice, increases as the A or B dimension (see above plot) increases, and vice versa. For example, if the B dimension is shortened from 50 cm to 30 cm (1 ft.) the required C value must be raised from 45 to 50 pf to counteract the reduced inductance and keep the trap resonant at 7.08 MHz.

It may be preferable for trap robustness to use a fixed C and a variable L. To vary L you slide the rods in and out of the tower or slide the taps for the A arm along B rods. Adjust both ends of A at each step so that the A wire is parallel to the tower.

If a variable capacitor is used you can opt to remove it from the trap after tuning is complete, measure its value with a capacitance meter and substitute a suitable transmitting capacitor of that value. Getting an exact match will be difficult so you should adjust the trap inductance afterwards to compensate. Whether a fixed or variable capacitor is used you must protect it from the weather to prevent damage and so that precipitation does not alter its value or breakdown voltage.

After tuning you must test the antenna from the shack. Confirm that the F/B has a sharp peak and that the SWR curve and resonance are as per the design. You can then complete tuning the of antenna, confident that the tower is no longer interfering with its performance. The trap should not need further adjustment after the antenna is tuned.

Variations on a Theme

Trap orientation -- As modelled the plane containing the trap inductor is orthogonal to the loop elements. While this is largely immaterial to the design and tuning of the trap there is a small affect on the antenna pattern. When the plane of the trap inductor is parallel to the loop elements the pattern becomes asymmetric near the pattern's nulls and rear direction. The effect isn't large so it can be ignored. However it does demonstrate how fine a balance between element currents is required to achieve a large F/B.

Vertical trap placement -- The trap can be moved up and down the tower, but does it make it difference? In my model I centred the trap on the tower, so that there is 6.5 meters of tower both below and above the 5 meters high trap section. I tested this by first moving the trap so that its bottom is 3 meters off the ground. This might be preferred to make it accessible from a step ladder.

I again tuned the trap to 7.08 MHz, and it turns out that the value of C is unchanged. I had suspected that by proximity to ground would alter the inductor value. Unfortunately the frequency of both maximum gain and F/B shifted downward by ~25 kHz and the F/B curve across the band degraded by several db. This isn't a large problem, but still. On the plus side the SWR curve improved! There is now another dip to 1.0 at 7.2 MHz (see chart). However the antenna performs almost no better than a single loop above 7.2 MHz.

Next I moved the trap higher on the tower, so that the top of the trap is 3 meters below the tower top. Interestingly the pattern and match behaviour was almost identical to that with the trap low on the tower. The significant differences are that the SWR is even better, staying below 2 even at 7.3 MHz. This time, to my surprise, it was necessary to raise the capacitor value from 50 to 63 pf.

In both cases the worse performance is visible in the currents plot. The longer section of tower outside the trap develops a higher current which interferes with the desired performance of the array. Of course if you're willing to sacrifice pattern performance (primarily F/B) for full-band matching you now have an option. However keep in mind that this is only a simple model, and once you add in the capacitive action of high-band yagis above the tower what you are likely to achieve in reality will differ, no matter which height you place the trap. My guess (which I won't bother modelling right now) is that with yagis in play the trap ought to go above the half-way point so that the electrical length of the sections above and below the trap are approximately equal.

Trap versus No Trap

Building, tuning and maintaining a tower trap requires effort, some expense and ongoing maintenance which, I believe, should only be undertaken if proven necessary. Since the resonance of the tower (plus ground and other attached antennas) is typically too difficult to predict it is best to try the vertical array (or just a single test element) first and determine whether the tower can degrade to the loop array performance. Do this by looking for anomalous SWR -- where the impedance curve and resonant frequency significantly departs from the model. Anomalies in F/B and gain are more difficult to discern.

If a trap is warranted don't hesitate to do it. When you go to all the trouble of building a large antenna to improve low-band performance it is unwise to ignore the signs that the antenna cannot perform as intended. This is too often easy to overlook (and convince yourself otherwise!) when a second antenna for the band is not available for comparison.

Even after you do build the trap you might not yet be done: solving one problem can introduce others. The placement of a trap for 40 meters in the tower could alter the performance of nearby 80 and 160 meters antennas by introducing a tower system resonance on those bands. As with any trap, on lower frequencies it acts as an inductive load which could lower the antenna system resonance to one of those bands, creating a destructive resonance that was not present before installing the 40 meters trap.

Of greater concern is any antenna for 80 or 160 meters that incorporates the tower as part of the antenna. Examples include half-slopers and shunted towers. At the very least those antennas will require retuning once the 40 meters tower trap is installed and tuned.