Admin note: VPR-02/2006 prohibits the use of v PMR band any external antenna. The article is only informative. In this second article in a row dedicated to the issue of PMR, we will get to know more about the HB9CV directional antenna, which I built and practically tested in the field, and in the following lines I want to share with you my experience with this antenna.
You will read in the article
Dimensions of the HB9CV antenna for PMR
Green dot - connection of coax shielding
Red dot – connection of the live part of the coax
We connect the tuning capacitor between the green and red dots
R: 337 mm
Z: 310 mm
D1: 298 mm
D2: 294 mm
and: 42 mm
b: 45 mm
R – Z: 84 mm
Z – D1: 84 mm
D1 – D2: 108 mm
We need
Cu wire: 4mm/0.5m – to the antenna beam
Cu wire: 2.5mm/2m – for antenna elements
Cu wire - 1.5mm/20cm - for phasing line
Capacitance trimmer – 2-20pF
Coax. Cable 50ohm eg RG58
Procedure
We will use 4mm copper wire for the antenna beam, we can choose this wire so that we still have a little bit left at the beginning of the antenna for possible fixing etc. We cut the necessary lengths of 2.5 mm Cu wire for the antenna elements, it is necessary to pay attention to the accuracy of the lengths of the elements, because all inaccuracies will be reflected in the gain of the antenna. On the cut wires for elements, e.g. with the help of a marker, we mark the centers, we also mark the places where we will place the elements on the beam wire. Using a resistance soldering iron, we solder the elements to the main supporting wire - a classic transformer soldering iron is not suitable because it cannot heat the wire to the required temperature. Next, we solder the phasing line, on which we use a 1.5 mm wire so that it bends easily. Finally, we solder the tuning capacitor and the coax. cable to the place as shown in the pictures and we can start tuning the antenna.
Antenna tuning
This antenna is one of those that must be tuned before use in order to radiate as much power as possible. We will carry out tuning as follows:
1) Using the elmag force measuring device. field, we place the antenna so that it is close enough to this meter.
2) We crank the PMR, watch the ammeter value and slowly turn the cap. trimmer until we reach max. value on the ammeter. I reached max. radiation power at a capacitance of about 5pF. I would also add that it is convenient when tuning the capacitive trimmer with something non-conductive, because when you use e.g. metal screwdriver, you actually increase the capacity, and when you pull it away, the deflection of the ammeter will either decrease or increase.
Use in practice
After tuning the antenna, it is good if you coat the antenna with a protective varnish, as it is known that copper oxidizes very quickly in the air, which has an adverse effect on the antenna, namely the skin effect - which means that at such a frequency, the current spreads only along a very narrow edge of the conductor. If you don't have any special varnish at hand, you can also use a mixture of rosin + alcohol. The antenna is used in vertical polarization as shown in the picture
Antenna testing
To test the antenna, we went to a nearby hill outside the city. We also heard some Czechs, but they didn't hear us. I would add that the nearest Czech border is approximately 80 km away. So we haven't heard from each other very far yet, but I can promise you that in the next few days we want to go to a bigger hill and try a couple of connections there.
Pictures of the finished antenna
Finally, I wish you the best of luck with your build! If there are any problems, let me know!
Article author: zawin
The article was published on the website http://svetelektro.com