Translated from Russian with Google Translate, Written by YL3BU. This page is a work in progress.
I am working on this text and format. Translating this text was more for myself because I wanted to make changes to my 4btv but didn’t find any similar information somehere else, so I had to translate.
The antenna is a quarter-wave trap vertical requiring either a good ground connection if mounted directly on the ground, or a good counterbalance system if installed in an elevated position above the ground. The height of the antenna together with the mounting bracket is approximately 5.7 meters. In the description of the antenna, it is proposed to choose one of the four proposed types of installation:
- On the roof, a mast 1.2 m high with counterweights;
- On a metal mast with counterweights at an angle of 45 degrees;
- On the ground with counterweights;
- On earth without counterweights.
|type of instalation||A cm||B cm||C cm||D cm|
For each of the options, the values A, B, C and D are proposed, which must be observed during the initial assembly of the antenna (Fig. 1).
The recommended counterweight lengths are also indicated. Two diametrically spaced counterweights are recommended for each range.
|Range||Counterweight length, m|
The antenna is tuned to a minimum SWR in sequence over the ranges 10 -> 15 -> 20 -> 40 meters, because the elements of the high ranges affect the setting of the elements of the low ranges. The setting is made:
- changing the values of A, B, C and D;
- counterweights – a selection of their length and location, isolation of the ends.
I decided to quickly model this antenna in MMANA . Everything seems to be there – both the dimensions of the elements and the data of the ladders, but for sure, as it seemed to me, the described 4BTV antenna model in MMANA did not work. More precisely, an acceptable SWR < 2 was only on the 10-meter band, where only the lower part of the antenna works (section 1), and the first trap tuned to this range “cuts off” everything else. But already on the next band, 15 meters, serious deviations began.
Section 1, section 2 and the 10 meter trap work on the 15 meter band. A 15m coil simply cuts off the top of the antenna. Thus, to obtain an acceptable result in the range of 15 meters, it is necessary to correctly describe the trap of the 10 meter range, or rather, the inductance of its coil. In my first model, I indicated the inductance incorrectly and began to shorten it – sawing the second section of the antenna, driving it into the range. I knew how the coil works and was sure that I could immediately determine its inductance and capacitance, but I didn’t take into account a lot …
… 09.01.2015 .
I decided to continue this note after 2 years. The antenna, in the end, was installed, configured and works well. Due to the lack of time (and, probably, the relevance of Hi), theoretical research has stopped, but still I would like to convey the thoughts that I came to. First of all, the similar and real design of the coil is not a point one – its electrical parameters are not concentrated at one point and are easily measured, but are complex quantities, consisting of several and affecting each other. However, this applies to any real design. Obviously, everything can be calculated theoretically, but the calculation of this coil will not be trivial. The calculation program for such a coil has not yet been posted (for coaxial, please). This is probably why there are not so many completely home-made structures of this type, more copies of something ready-made. Yes, and the wires, between the ladders of coaxial cable, are easier to cut than sawing and sharpening pipes with meters, Hi. Therefore, further a little about the design and some facts.
Below is an open trap of the Hustler 4BTV antenna (at 14 MHz), as well as a sketch of it to understand the device and the principle of operation. The design is very simple. The coil is wound on a dielectric rod – an insert (which also carries the main mechanical load) between the top and bottom tubes of the traps and is connected to them. Above the coil there is a sleeve that protects the coil and, at the same time, is the main structural element .part of the tank capacity. In the lower part, the sleeve has a narrow neck with two diametrically located slots on which a clamp is put on to fix the position of the sleeve relative to the coil. The slots are not hermetically sealed and serve to ventilate the coil. The upper part of the glass is closed and fixed relative to the upper pipe of the trap with a plastic cover. During final assembly, a sealant is applied under the cover, so the upper part of the drain is airtight. The sleeve can be moved up – down (right – left in the figure, respectively) when adjusting the ladder at the factory or when adjusting the ladder when installing the antenna. And if you, after all, decide to adjust the ladder yourself, you lose the warranty. In my case, not having achieved the proper result according to the instructions, I had to adjust the traps of the 21 and 14 MHz bands.
It is not recommended to tune the antenna by changing the length of the supply cable and using a tuner, as this does not improve the radiation efficiency of the antenna, but only makes life easier for the transmitter. It is possible to adjust the ladders, but more on that below.
The trap (parallel oscillatory circuit) consists of a coil and a capacitor.
Coil . Having opened the trap, the inductance of the coil can be measured with an LC meter and / or calculated by the formula by measuring the size and number of turns. But this will not be the actual inductance of the coil. The coil is close enough to the top and bottom tubes of the trap, but this is not the main influence. A sleeve placed on top of the coil, and located close enough to its turns, is a screen and reduces the inductance of the coil. Moreover, the coefficient of influence of the screen depends not only on the physical parameters of the coil, the screen and its location, but also on the frequency, Hi. In one smart book, I found formulas and graphs for calculating this effect, drove everything into Excel, tried it … well, somewhere, somehow it approximately coincides, but in the end the trap did not “calculate” for its native frequency, Hi.
Capacitor . At first glance, it is also elementary, above its lining are drawn in blue and red. Obviously, the main “share” of the container is formed between the upper (right in the figure or red) tube and the sleeve, which is electrically connected to the lower tube (drawn in blue). By moving the sleeve up (to the right in the figure), you can increase the capacity, back – decrease accordingly. Thus the trap is configured. But we should not forget about the interturn capacitance of the coil itself, as well as the capacitance between the turns of the coil and the glass, which are also included in the total capacitance of the trap capacitor.
Measurement of the inductance of the traps coil of the 10 meter range with a homemade LC meter directly and with the sleeve installed in the working position.
Measurement of capacitance between the top tube and the sleeve of the trap of the 10 meter range.
General view of the traps with lowered sleeves.
Measuring the resonance frequency of a trap of this design using an antenna analyzer is obviously not a simple matter either. Unlike a coaxial trap, the assembled coil is hidden under a sleeve and there is simply nothing to “connect” the antenna analyzer’s communication coil. I tried to place the communication coil inside, and then assemble the ladder and measure, incorrect measurements were obtained. This is another disadvantage of this type of traps.
As I wrote above, I did not finish my “scientific” research and the antenna was installed above the elevator booth on the roof of a 9-storey building. A description of this process and the results can be found here . Everything was done according to the instructions for the installation option on an additional mast. But, as you can see from the specified link, the 21 and 14 MHz bands could not be tuned more beautifully, the resonance frequencies were outside the ranges. Adjustment with counterbalances made it possible to change the resonance, but the reactive component of the input impedance of the antenna grew and the SWR only worsened. It was October, the weather was getting worse and I no longer climbed onto the roof. Worked for a year through 25 meters of RG-213 cable using the automatic antenna tuner of my TS-480SAT transceiver and losing up to 20W on the 14 and 21 MHz bands according to calculations .
A year later, having collected my thoughts and calculations, I removed the antenna for repair and tuning.
Repair. I have not read about such an experience anywhere, but fat fat Riga cormorants, sitting on capacitive load tubes, broke them off. Of the six tubes, only two remained alive, and then bent down to the ladder of 20 m range. I found one of the tubes near the house on the ground, Hi. New ones were installed, of a larger diameter (it seems 12 mm) with a wall of 1 mm. Moreover, a thick fishing line is stretched between each tube and a 40 m emitter so that the birds do not land. It wasn’t there. On the fly they squint, obscenely harsh, but sit down under the fishing line closer to the antenna. But the upgraded version of the capacitive load has been able to withstand this for 2 years already.
I also took additional measures (just in case) to protect the ladder from moisture. The joints of the plastic caps at the top of the drain were wrapped with plumbing tape. The bottom of the ladder with a longitudinal slot of the squeezing cup is not sealed. The slot is designed for ventilation of the ladder.
Setting. I read somewhere (it seems in a special recommendation from “Tangenta”) that at the Hustler factory, antennas, namely their traps, are configured for installation on the ground and with unconfigured counterweights. Maybe now it’s not so, but I got, obviously, this option. My setup, shown here, is on a mast with counterweights suspended high. Therefore, having some experience in calculations and experiments, I decided to set up traps. As a result, what was done with the antenna, to tune in given conditions, in order to obtain acceptable parameters for the bands (with distances A, B and C according to the instructions with the installation on the mast):
- 10 meters. Nothing but a little tweaking of the raised counterweights. SWR is acceptable over a wide frequency range.
- 15 meters. The 15 m sleeve of the trap was shifted 6-7 mm down (reducing the capacitance and increasing the resonance frequency). The SWR minimum moved from 20.8 MHz to the CW section of the range, as I wanted. Adjustment of raised counterweights.
- 20 meters. The sleeve of the 20 m ladder was shifted down by ~ 9 mm. The SWR minimum moved from 13.7 MHz to the CW part of the range. Adjustment of raised counterweights.
- 40 meters. Here is about the distance D, or more precisely about the length of the top (Sect.4 in the figure above) of the emitter on the 40 meter band. From the previous owner, I received this tube “Sect. 4” not original, but about 10-15 cm longer (sorry, I didn’t measure exactly, I installed the antenna right away), otherwise the resonance in this range was higher in frequency. Adjustment of raised counterweights.
After working with this antenna for more than a year, I noticed that with heavy precipitation and high humidity, and especially with strong winds with sleet, the resonance frequency shifts downward. For example, on the 20 meter band, if there was a minimum of 14.050 MHz, then with a strong wind with snow, there is a shift to ~ 13.900 MHz. If I had climbed onto the roof with preventive maintenance, I would have rebuilt the 15 m and 20 m ladders not at the beginning of the CW range, but a little higher, so that this “weather” offset of the resonance walked, if possible, within the range. To do this, as already mentioned above, the glasses of the ladders must also be moved down by ~ 5 – 7 mm (in my case).
That’s all. 73!