[TowerTalk] Lossy Traps (really long)

[Tony Brock-Fisher]

The issue of lossy traps is only one of a number of factors that enter into antenna comparison and selection. The real question is which antenna is best for you? And of course, there is no simple or single answer. Over the period of more than 20 years, I have considered this question many times, and I have consistently come up with a single answer for my station. So before I continue, let me admit to being a fan of the KT series of antennas - the original KLMs and the newer M2 version. However, that being said, I'd still offer the following for consideration.

 

Lossy Traps

 

Traps are seen to be lossy for two reasons - one is the issue of limited Q and thermal losses of the input power. A second issue is that traps inherently result in shortened elements, which results in slightly reduced gain.

 

Heat Loss

 

Several years ago, when this issue was raised before, I had an opportunity to make some measurements on the thermal (resistive) losses in the traps of the KT antennas. Now, the KT antennas are often referred to as having 'linear loading' and not traps - but this is not really true, as on 10 and 15 meters, the combination of linear loading inductance and air-capacitors does form a tuned circuit. It may be the lowest possible loss for a tuned circuit - but it's still a trap. I did some careful measurements, which I will reprint here:

 

 

(Republished from 1996)

 

In an attempt to get a handle on this subject, I recently made some

measurements which would serve to give some idea as to the truth of

these arguments.

 

Part of this summer's antenna projects at K1KP involved removing a KT-34XA

from the top of my tower while the tower was being rebuilt. I've had

the XA down on the lawn for most of the summer.

 

This particular antenna was first purchased as a KT-34 in 1983. I purchased

it used from the original owner in about 1988, when I did the first rebuild

on it. I added the XA kit and did another rebuild, in 1993. So when the antenna

came down early this summer, it had 3 seasons on it. It was working properly

on all bands.

 

[details of the rebuild omitted]

 

Next, I attempted to quantify the power loss of the traps as follows:

 

I set the antenna up on my driveway on sawhorses. Although the antenna

was only 30" off the blacktop, it had reasonable SWR on all three

bands. I fed the antenna with 100 feet of RG-8/U cable, whose loss

was within spec.

 

I used a Fluke 80T-150U temperature probe, connected to a Fluke handheld

DVM to measure temperature. All measurements are in degrees Fahrenheit.

During the experiments, I measured temperature at three locations - 

the strap connected to the 10 meter capacitor on the front driven

element; the boom, just behind the front driven element; and the PL-259

connecting the feedline to the supplied KLM balun.

 

I measured the boom temperature as a means of watching for changes in ambient

temperature. I made the measurements starting at 5:30 pm on a nice warm,

sunny day, so the sun was going down and ambient was dropping slowly.

 

First, I made initial measurements, then gave the antenna a short blast

on 20 meters using my IC-765 driving and AL-1200. The RF applied

was 1200 watts for 5 minutes. Interestingly enough, the temperature on the

outer case of my AL-1200, just over the tube anode, rose from 91 degrees

to 253 degrees!

 

            Boom      Trap      Balun

Initial     85.4      81.7       85.8

Final       85.3      83.2      126.1

 

So the conclusion here is that the ambient decreased slightly, the trap

dissipated some heat, and the balun got warm. No surprise here. Next,

I added a piece of foam pipe insulation around the outside of the

10 meter and 15 meter capacitors. This would serve to reduce measurement

errors due to air movement cooling the traps. I could easily measure the

temperature by poking the temperature probe through a small hole in the

insulation. The insulation would allow the heat to build up for a more

accurate measurement.

 

For the next run, I applied 1000 watts of RF on 10 meters for 5 minutes.

I figure the 10 meter traps, being parallel resonant on 10, should have

some pretty big circulating currents in this mode.

 

         Boom     Trap     Balun

Initial  82.7     81.8     107.8

Final    81.0     87.2     178.0

 

So in this mode, the trap temperature rose 5.4 degrees. If you include the 

fact that ambient dropped 1.7 degrees, this is a net rise of 7.1 degrees.

I'm not sure the ambient decrease needs to be factored in, as the insulation

on the trap should have prevented it from being cooled; however this will

lead to a higher dissipation estimate so I will let it stand.

 

Next, in order to quantify the amount of power that 7.1 degrees represents,

I installed a 10 ohm power resistor inside the foam, in physical contact

with the 10 meter capacitor tube. I applied 7 volts DC across the resistor

for 5 minutes, and measured the temperature rise as before:

 

         Trap

Initial  71.2

Final    83.4

 

This shows a 12.2 degree rise due to the application of 4.9 watts for

5 minutes. If we assume that the ratio of temperature rise to power

dissipated is linear, this  means that 2.85 watts were dissipated in the trap.

 

Now let's extrapolate this measurement of power dissipated in a single

10 meter capacitor to power dissipated as heat in the entire antenna.

I'll do two scenarios - conservative and optimistic. First the conservative:

Assume that I only accurately measured half of the power dissipated in the

trap, i.e. that a similar 2.85 watts was being dissipated in the inductance

portion. Also, assume that the same amount of power was dissipated in

all of the ten traps of the antenna. This results in a total power dissipation

of 57.03 watts. If the antenna was fed with 1000 watts, the efficiency is

94.3%. Or expressed in dB, the resistive losses were 0.25 dB.

 

Now for the optimistic model: Assume I did measure all of the power

dissipated in the trap. Also, modelling tells us that in this antenna,

the element currents are not all equal. The front driven has the highest

current, the rear driven has somewhat less current, and the parasitic

elements have much less current than the front driven. So instead of 

multiplying the power in one trap by the number of traps, we need to

multiply the current in the measured trap by the current ratios given

by modelling to get the current (and power) in the other traps. This

results in a total power dissipated of 12.19 watts; efficiency 98.7%;

resistive losses of .05 dB.

 

(End of Republished Text)

 

 

Gain Loss due to element shortening

 

When traps are added to an element and operated at a frequency below the frequency for which they are tuned, they insert an inductive loading into the element. To compensate for this, the element is typically shortened to bring it back into resonance. When a half-wave dipole (the basic element of a beam antenna) is shortened, it's pattern becomes broader and its maximum gain drops by a few tenths of a db. The overall gain of an array of elements is the product of the gain of each element times the gain of the overall array. Therefore the slight reduction in gain from a shortened element carries over to the array of shortened elements. I haven't done the modeling lately - but I think I recall that in the case of trapped antennas the gain reduction is on the order of tenths of a db.

 

Apples vs Apples comparisons

 

Many antenna designs exist which achieve multiple band functionality without traps. One means of achieving this is to simply combine two or three monoband beams on the same boom. The downside of this approach is that the elements of the lower frequency antenna tend to disrupt the pattern of the higher frequency antennas because their long (and often harmonically resonant) elements act as reflectors in the middle of the higher frequency antenna. The way around this is to forward-stagger the elements - which requires more boom length, but allows the different antennas to coexist on the same boom with less interaction. The C31XL and Bencher Skyhawk are examples of this. The problem comes when trying to compare these antennas to conventional trapped tribanders. Without resorting to modeling, most hams realize that boom length is the primary determinant of gain in a beam antenna, with the total number of elements being less of a factor. So one might compare the 32' boom of a KT-36XA to the 31' boom of the C31XL and conclude they are similar in gain. However, due to the forward stagger, none of the three monobanders on the C31XL boom get to use all of the 31' boom length - with a resultant reduction in gain.

 

Reliability & Other Factors

 

Recently I had to do a rebuild on my KT-36XA. (Anyone who has read this far and is still awake and interested can email me for the write-up of the rebuild). Because of the expense and time involved in this project, I seriously considered changing to the nouveau sine qua non antenna, the SteppIR. Before I made the jump, I asked several local owners about their experience with the SteppIR. I was especially concerned with reliability, given that it has moving parts. This was a major concern, right up there with forward gain. What I found was less than encouraging. Several local folks had had problems with various parts of the antenna, including cracking/failing boots, jammed or broken motors and gears, and many had control box failures due to lightning. So given my high concern with reliability, I chose to stay with the M2, and did everything I could to improve it's reliability. 

 

I will state that I think a properly working 4-element SteppIR on a 32 foot boom probably beats the XA in forward gain, due to slightly lower losses and full size elements. However I was surprised that the manufacturer is still experiencing material and vendor problems. I would have expected that they would be on a campaign to address the number one concern of the SteppIR design - reliability of moving parts.

 

The bottom line is that one should not get to wound up about trap losses, and that there are many other factors that can outweigh the trap issue when making an antenna selection.

 

Respectfully Submitted,

 

Tony, K1KP