Help a newbie with his first antenna

[Taphorn]

Hello!

I've been into simple hobby RF for a while, but I'm just recently getting into antenna design. I got into RF stuff when I bought my first SDR receiver (those cheap rtl-sdr USB dongles on ebay). They work from 24 MHz to about 1,7 GHz. I own some better SDR devices today, but I've mostly worked in the VHF/UHF regions. Anyway I decided to build my own antenna. Since I'm already working on a RaspberryPi project to transmit sensor data over the 433,92 MHz ISM band, I'd like to build an antenna for that frequency. I originally wanted to build a dipole, but I've been told to rather focus on a more convenient HB9CV antenna. It's basically made of small soldered brass tubes of certain lengths. I've already built the metal part by consulting the online HB9CV calculator (with some tweaks), but I have yet to tune it - a thing I've never done before.

Here's what I came up with:

I've read some online docs, and I'm gonna share what I've learnt just to verify I'm on the right track. The docs usually state you have to minimize a parameter called VSWR. Afaik this parameter tells you how well an antenna is matched with other components regarding their impedance. Anything below a ratio of 1:2 would indicate a good match while anything above 1:6 likely means a bad match. However most don't say how to change the VSWR or adjust the antenna impedance to match 50 Ohm. Some suggest to add an appropriate matching balun instead, but I'm not sure that's what I want here.

I've read there are other parameters that need to be tuned. The resonant frequency should match that of the transmitter/receiver and it can (in my case) be adjusted by the adjustable capacitor at the antenna feedpoint. A few docs also mention reactance/inductance, and if I remember correctly it goes like this: When inductance and reactance cancel each other out, the antenna becomes purely resistive load - it is said the antenna is at its resonant point.

So tl;dr, I have to tune the following parameters:

• VSWR (lower ratio = better)
• Impedance (50 Ohm)
• Resonant Frequency (reactance/inductance)
• Any other I've missed???

Apparently I'm going to need some measuring hardware for this. I do have some existing hardware though: a cheap swr bridge from ebay, a random noise generator from ebay, various SDR receivers, some SDR transmitters (i.e. HackRF), a Rigol oscilloscope, a simple function generator. I don't have much knowledge in DIY RF circuits and even less in calibrating them. Should I just buy a dedicated antenna analyzer? Which one for the VHF/UHF regions? I'm a pleb so I don't have the money to buy expensive hardware in the several grands range. What are your suggestions?

Additional questions:

• Since I will be transmitting on this antenna, should I also add a 1:1 current balun (common mode choke) at the antenna feedpoint? Which type?
• Out of curiosity, what is the maximum transmit power an antenna like this can handle? Though I'll only be transmitting at a few milliwatts, I've seen similar antennas being sold with 100 to 200 Watt TX power.

Thanks for any help!

 

[majoco]

The QSL article explains all.

You need a variable airspace capacitor at the feed point which you tweak for best VSWR then remove the variable and replace with the closest value fixed capacitor. If you want to transmit with high power then I would think that you should keep the airspaced cap and put it in a watertight container somehow. It's a pity you can't get those 35mm film cans anymore!

HB9CV

 

[Taphorn]

I'll just get a dedicated antenna analyzer for now. A lot of people seem to be recommending N1201SA for about 170 bucks from banggood; or maybe I'll even get the higher end PocketVNA. The latter price is technically not in the thousands, but it does have some extra functions, wider freq range and also does smith charts. We'll see. The Rigol oscilloscope and the function generator I've mentioned unfortunately don't go into the VHF/UHF range after checking their specs so they're out of the picture.

The QSL article explains all.

I'm not so sure...

• Where does one attach the HB9CV antenna feedpoint - at the radiator or reflector? The article you linked does it at the radiator while the online calculator I linked in first post does it at the reflector. What's the difference?
• Do I need to add a common mode choke to the antenna feedline for transmitting? If so, which type? Some people say just add a solenoid (wirewound coax), but some hams argue they don't really work and that inductive/lossy ferrite choke should be used instead.

 

[majoco]

The sketch and the photo clearly show the feed at the centre of the short element, but other articles show that there is no director/reflector as both elements are the same length.