Matched long-wire antenna by RU3AEP
Building an efficient antenna is a big problem for an radio amateur in many cases. HF antennas, especially for lower bands, have large sizes and have to be mounted on a big height. In many places, especially in the city, there is not enough room to erect the full-size dipole or 'inverted V' for lower (160 and 80 m) bands. Also, large antennas cause a long feeding cable to be used, and this is not good in terms of cost and construction: antenna with 'central' feeding should bear a weight of a cable, the feeder should be fixed somehow to protect all the system from strong winds, there is not always conditions to make whole antenna system suitable for the environment.
When i started to think, what antenna to build for a top-band (160 m), i realized, that condition are too bad for it. I live in a 7-floor house, which has a roof with a high slope (about 35-40 degrees). Such roof is very dangerous to operate on it. Also, the house is almost completely surrounded by wide streets and electrical wires going along them. After long thinking, i concluded, that there is only one possibility to make an antenna - to hang up a long wire from my roof to the roof of another house. If i had built a dipole on that place, i would have had too many problems with the feeder. Any other places of antenna mounting were unacceptable from viewpoint of safety (antenna should never hang above electrical wires!!!, otherwise in case of fall down in may injury or kill you or somebody else and also cause severe electrical damage of the network and your equipment!) or from viewpoint of difficulty in mounting.
An old idea which is actual today for many city HAMs
Fortunately in that time i have read about one very old, but not frequently used antenna - so called Zeppelin-antenna with a matched feeding. Classical design is presented below. As it can be seen, there is feeder with rather low impedance (~70-300 Ohm), and 1/4-wavelength matching line. From one end, this line is shortened, and here its impedance is just a zero (current is high, but voltage is low). Another end of this line is connected to the long wire, which has length exactly 1/2 wavelength. At this end, the impedance is very high (several kiloohms). That is why, a big voltage exists here during a transmission. Such impedance is quite suitable for a wire feeding, because a 1/2-wavelength wire has voltage maxima (high impedance) at the ends. The feeder from the transmitter with a specific impedance R(feeder) is connected to the matching line in the point, where impedance of the latter is equal to that of the feeder. Such point is usually located not so far from the closed end. If everything is done properly, feeder may have any length and SWR is closed to 1 in rather narrow band, central frequency of which is determined by the geometrical size of matching line and antenna.
This design can be used almost without change, but instead of symmetrical feeder coaxial cable can be used to connect the whole system to the unsymmetrical output of the transmitter. The using coaxial cable has also additional advantages- it is almost insensitive to the environment, can be placed everywhere and is very flexible.
Such antenna with feeding 'from the end' is much more easy to make, that a simple dipole. Here, antenna conductor bears only itself, and this reduces the mechanical loading and thickness of the wire to be used. Also, you may use your window as one the point of antenna fixing. In this case, all the cable will be inside the room and could be tuned precisely in comfortable conditions. If the beginning of antenna is outside the apartment, most part of matching line can be used as the continuation of the feeding cable. On the next picture there is a design, that i implemented for using on 160 m.
All coaxial cables have 75 Ohm impedance, the antenna wire, as well as two bearing wires are made from very hard bimetallic insulated cable (outer diameter is about 3 mm). The most tricky part - the connector between cable and antenna - is shown on the picture. It should be noted, that voltage on it is quite high, so everything should be well insulated from each other. It is good to place this connector somewhere indoors (not far from window/hole), otherwise rains and snow may cause decreasing of insulation efficiency. This antenna uses a tuned line made from the coaxial cable, and for proper operation of the whole system the antenna should have the length equal to the l*0.95/2, and the coaxial line must resonate on the working frequency. It is a good idea, to connect the shortened end of the matching line to the ground to provide adequate safety and to reduce possible TV/RF interference while transmitting.
Tuning of the antenna
To achieve what was declared in the previous paragraph, first of all the precise length of the matching line should be determined. Theoretically, it should be equal to l/(4*sqrt(d)) (sqrt - SQuare RooT, d - dielectric constant of the insulator used in the coaxial cable). SQRT(d) value is typically about 1.52 for most cables, that is why, 'shortening coefficient' is about 0.66 (1/sqrt(d)). But the practical value will be a little different from that.
The lengths indicated on the picture are mine values, and they can be used as the approximate reference. To make your line resonate on the middle of the band (1890 kHz), you have to make the line about 1 m longer, that indicated on the picture (for example, 24 m). Then, connect the 1-2 kOhm resistor to the end of the line, and the transceiver trough SWR meter - to the feeder. Put some power into line and watch the SWR. If the line is completely out of resonance, SWR will be closed to infinity, and no power will be dissipated on the resistor. Then the frequency should be found, which gives the sharp minimum of the SWR. It should be somewhere 1750-1800 kHz. Here, the SWR should be no more than 1.5. After the resonance have been found, the end of the cable should be cut carefully in several steps, watching the resonance frequency each time. By the cutting, the resonance will be shifted up. After you achieve the desired frequency, your line is almost ready, and you can mount the antenna in the chosen place. The minimum of the SWR in mounted antenna is usually 10-15 kHz down, compared to the value achieved by the tuning. If the SWR in minimum is too great - the point of the feeder connection should be varied to achieve acceptable value (it means redistribution of the cable between short and long sections of the line - not easy task).
When i made the antenna by the way described, everything was OK, and i had a minimum of SWR at 1875 kHz (about 1.3), on the edges of the band SWR increased to 2.2-2.5, since this antenna is a narrow-band one. Compared to my previous dipole, which hanged on the low height along the building, this antenna exhibited much better transmission efficiency and higher signal/noise ratio while receiving. But unfortunately, nothing lasts forever, and this is not an exception. Having read the next section, you probably will understand, which problems are encountered by the ham operator in a big city like Moscow.
NEW! (it would be better, if it doesn't exist)
- A short and real story about being an amateur radio operator.
Some recent bad news....
It took a lot of efforts to me, to built the antenna system described on this page. The main problem was quite usual for people, who live in big cities: the district's administration did not want to allow me to build something, and the reasons were, that I had no authorization to operate on a height, that my constructions would damage the roof and so on. After about twenty visits to different offices and talks with employees i got the official permission to build what i wanted.. The rest was quite simple - I found all necessary materials and made all parts of my future antenna. One weekend with a help of my friend I mounted all wires and cables and, being happy, started to operate. . .
Although everything went quite well with this antenna for about a half-year, one day I turned the receiver on and did not hear any signal.... Having looked out from the window I saw, that my coaxial feeder, which had always hanged from the underroof space to my apartment, was cut off somewhere and hanged between 5-th and 2-th floors of the building. You can imagine, which feelings I had that evening...
Next day I came to the underroof space and suddenly saw, that some asshole cut the cable in two places, but fortunately the potential repair was rather simple. In a week I repaired it completely, but this was not for a long time - after three days the situation repeated, but now the simple repair was not possible (if any repair was reasonable in such situation) - the feeder with matching line was completely distorted by unknown 'knife-terrorist', the matching line was cut in many places, and the pieces of a coaxial cable were lying around - terrible view! I could not even find the 'T'-shaped jack, which served (unfortunately in the past!) as the connection between the feeder and matching line. Probably it was stolen or just thrown away into a lot of trash in the underroof space, but anyhow I feel, that building and using of large antennas will be difficult and may be unreasonable for me, taking into the account such wild people and inability to protect the antennas from such vandals.
I heard many times, that amateur antennas are objects of vandalization in many cases - people sometimes have TVI/RFI problems (which are not directly related to the ham station in many cases), and instead of speaking with the ham operator they prefer to take tools and destroy what they can, and what, to their mind, is the source of their problem. Even without any particular problem, some dummy persons just do not like (due to their stupidity they could not even imagine, that there may exist another interests, except eating, drinking and watching TV), that somebody builds something 'strange', and make all attempts to create problems to the owner of these constructions. I can not understand them, but such people still exist.
May be my transmitter was the source of interference for somebody, I can not exclude it taking into the account the absence of the ground lines in our house (it is quite old), but I think, that it is really unfair to solve these problems by such kind of the acting, and it is really strange, that this happened after about half year of almost everyday working (I had been making at least 10 QSOs every evening, sometimes speaking more than a half an hour with one correspondent) . I would never desire somebody to have such kind of the problem and such kind of neighbors.
The whole situation can be called only by one expression -it is in URL. If you did not come directly to this story by the link, go to the beginning of the page, follow the link there and look at the exact URL in the address field of your browser. Have fun with your antennas and read next paragraphs!
The continuation of this story
As I noticed above, nothing lasts forever, even a disaster. Having well thought, I decided to repair the whole system again, using self-made connector to restore the cable. Also, I connected the end of the matching line (which is shortened, see the picture) to the nearest water supplying pipe, which are in excess in the underroof space. I think, this provides a permanent grounding of the matching line, yielding higher safety and less interference. I guess, my thought was right - since that time nobody cuts the cable more, and I am sure, that my antenna has always the ground potential, which is quite important in thunderstorms. It is really good, that the problem seems to be solved.
73!, Valentin Gvozdev , RU3AEP.