Indoor magnetic antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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For someone who is forced to abandon an outdoor antenna (for example, after moving), things are very bad for a shortwave radio amateur. However, something can always be done to still remain QRV.
A typical solution is a magnetic antenna. Despite satisfactory experience with two homemade indoor loop antennas for 80...10 m bands, there was very little hope that something similar could be done for 160 m.
Based on the well-known Annecke sketch and the loop made from it for 80...30 m, I just wanted to increase the number of turns to four (double). Then the 400 pF capacitor that was used before could be left unchanged.
But this is a theory. What if it fails? Four turns of copper tube with a cross section of 18 mm and a coil diameter of 17 cm - isn't it too expensive for a mistake? After some thought, I came up with a not very expensive solution - to use a thick coaxial cable. Total consumption of materials:
- 4 wooden planks 20 mmx40 mm;
- 12 m of RG-213 coaxial cable;
- 4m coaxial cable RG-58;
- 1 capacitor of variable capacitance 100 pF with a distance between the plates of 2...3 mm;
- 1 coaxial connector.
How the design looks like is shown in Fig.1...3. A cross of wooden planks with cross planks, in which I cut grooves, holds a resonant loop consisting of four turns of RG-213. The upper strips (for the resonant loop and the communication loop) have two holes at the edges to hold the ends of the cable and three grooves between them. The length of the side of the coaxial cable in an almost square suspension is 67 cm. To those who are very bothered that I do not give the dimensions of the wooden cross, I must say that I respect old Pythagoras very much and that, strictly speaking, only the dimensions of the coaxial cable are important.
The screen sheath of the coaxial cable forms the frame of the loop. The cable core is broken at a distance of 3,50 m from one of the ends. The result is a capacitance of about 353 pF connected in parallel to a variable capacitor.
A variable capacitor is connected to both ends of the RG-213 cable sheath. For all connections and connections, a thick copper wire with a cross section of at least 10 mm must be used.2.
For reasons of convenience, the coupling coil is placed on the same strips and also has four turns at a distance of 8 cm from each other. The length of the side is 20 cm here. The RG-58 cable is connected to the input coaxial connector with both ends of the braid.
At resonance SWR = 1,2. The SWR<2 bandwidth is less than 9 kHz. Therefore, it is necessary to adjust the variable capacitor to resonance when tuning over the range. Perhaps this is the only drawback of the magnetic loop!
After going on the air for 15 days in a row (in January 1995) in the morning and in the evening for about one hour, I made 43 QSOs with stations from fourteen countries: DL, SM, SP, G, GL, GM, F , ON, 1, OK, OE, ON, 9A, S5. Unfortunately, I couldn't call many of the stations I heard because they purposefully called "CQ DX". Yet my own CQs only resulted in a few QSOs.
The existing noise level can be reduced by 2 ... 3 points by a slight change in the location of the antenna and its rotation. For a range of 160 m (and 80 m) - this is a true boon!
Since the antenna zeros (more precisely, almost zeros) are very narrow, the orientation of the antenna is completely uncritical.
In conclusion, I would draw the following conclusions:
-
it's not a super antenna, but I'm QRV again on 160m;
-
costs and space occupied by the antenna are negligible;
-
as a high quality symmetrical antenna, it greatly improves safety with respect to BCI/TVI (no RF stray currents);
-
atmospheric interference and local QRM are reduced;
-
accommodation does not depend on weather conditions.
When the antenna is transmitting, do not touch it, otherwise you may get burned.
Editorial note
Instead of the RG-213 cable, any 50 ohm cable with an outer diameter >13 mm can be used.
Literature
- CQ DL 12/96.
Author: W. Shreglman (DJ5QY); Publication: N. Bolshakov, rf.atnn.ru
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