Please someone assist

I have a wire using as an antenna

How to make it's impedance 50 ohms?

This wire is connected to a capacitor then to an RF amplifier (bandpass after amp)

Thought I could place a 51 (50) ohm resistor to ground across the wire so the amp sees 50 ohms

But not sure this is right... The coax connector people use makes the antenna 50 ohms?

Understand the question?

Please assist

Thanks
73

 

The RF amplifier is designed to work into a specific impedance which is generally 50 Ohms so what that means is you generally impedance match meaning a 50 Ohm output works into 50 Ohm cable like RG 8 or RG 58 to name a few and that impedance matched coax in turn feeds a 50 Ohm matched antenna. No, you do not place a 50 Ohm resistor across the output. I just really over simplified things. Transmitters are tuned to antennas and the idea is have all the power transmitted and not reflected back to the source (SWR Standing Wave Ratio).

Ron

 

You don't say what kind of antenna but typically a Balun is used. It takes an unbalanced impedance (antenna) and matches it to a balanced (50Ω coax/radio antenna input) impedance IF that is what your radio/tuner input actually is. There are other impedance value baluns and ununs. Unbalanced to unbalanced. Just what do you have??? You need to do some basic antenna research and there are several old ARRL antenna books online as PDFs.

 

You need to do some basic antenna research and there are several old ARRL antenna books online as PDFs.

This ^^^^^

Since you used 73 you should be familiar with the ARRL.

Ron

 

Thanks thanks

The black wire on the left is the antenna

is it the connector that makes it 50 ohms?

 

Looks like what I assume is RG 58 50 Ohm coax. No, it's not just the connector, there is much more to it. I see BNC connectors which come in a variety of impedance. Google 50 Ohm BNC Connector, Then Google 75 Ohm BNC Connectors they look the same but they aren't. Read this and then read some of what SAM suggested as to ARRL Handbooks. None of what you are asking can be answered in a single post or paragraph.

Ron

 

is it the connector that makes it 50 ohms?

No, it isn't. Look on the cable jacket for a type marking. It should be stamped every few feet on the jacket. What I see, judging by its diameter could be RG-8X, RG-58, or possibly RG-75 or RG-11. All have different datasheets and specifications. Look them up! Most modern HF radios are 50Ω but find the datasheet for the radio to confirm. The silver-plated (not chromed) Amphenol PL-259 HF connecter used to be the "gold standard" and BNC for VHF. The RG-8/U 0.4" OD was often used but not as flexible as the small OD stuff now available. Still used for low loss and 2KW+ amplifiers. Now, type N low insertion loss connectors are becoming very common and many different brands. Some of the chinesium knockoffs are pure junk. Copycatted with no engineering to back up the design and zero quality testing for type compliance to the standard. Before you start transmitting and get locked up and fined big bucks in penalties for pirate radio, get a license or use a dummy load. Then we can talk about SWR and Resonance tuning and antenna matching and resonant antenna lengths.

 

Sounds like you have an end fed random length wire as the antenna. To match that to 50 ohms would require a wide range antenna tuner. But for a receiver, it is normally sufficient to just connect the long wire to the input of the RF amp. As long as the RF amp is stable with the random source impedance, it usually works well enought - not optimum, but good enough.

 

To lower the loss in the network made up of the cable, antenna and ground matching is very important.
yes the cable is 50 Ohm and the output impedance of the radio?
A diode bridge measures the highest voltage (sometimes called an Avramenko plug). It is trimming or tuning.
A resonance condition is preferred so weak signals will not be attenuated before they reach the receiver.
A resonance condition is preferred when a signal is transmitted. So the coax designed for 50 Ohm at the correct length will be close.
The standing wave will exist stronger if you provide a favorable length. It's full wave length might be accommodated.
Often the ideal is not practical, 3/4, 1/2, 1/4, 1/8 wave length might be almost as good and will serve other bands when they are multiples.

 

The simplest possible antenna is the end-fed long wire. Simply as long a piece of wire as you can, as high as you can between 2 insulators. The "trick" is to match the antenna impedance to the radio's impedance. It is done with a wrapped toroid coil such as pictured here: https://www.ebay.com/itm/HF-antenna...nd-dipole-antennas-10-30-MHz-5kW/252787549946 Note that this particular one is rated for 5kW. The reason to use coax is to shield the signal from the noise generated in the house. On the balun/unun it has a coax connector for the coax cable to the radio. The other end has 2 connectors, one for the insulated antenna wire and another to go directly to a driven ground rod. No, you don't have to go to these extremes but a good antenna greatly improves reception. Money spent on a proper antenna is a far better investment and cheaper than spending money on a better radio. If you look around on eBay you will find very inexpensive baluns and other antenna building parts. I have one end-fed 45' sloper just for my all-band receiver radio. I had a much longer one that came down during a hurricane and is still in the planning stages to rebuild and rehang. It will be over 100' long at 10-20' elevation above grade. I also have a couple other 10-80 meter bands off center fed dipole and 10 meter vertical antennas for my transceivers.

Something else you might want to consider is lightning suppression on the antenna feeds or simply remove them from the radio when not in use.

 

Please someone assist

I have a wire using as an antenna

How to make it's impedance 50 ohms?

This wire is connected to a capacitor then to an RF amplifier (bandpass after amp)

Thought I could place a 51 (50) ohm resistor to ground across the wire so the amp sees 50 ohms

But not sure this is right... The coax connector people use makes the antenna 50 ohms?

Understand the question?

Please assist

Thanks
73

You have not mentioned if this is a receiving antenna or a transmitting antenna, nor anything about the power level involved. That tends to affect the choices about impedance matching. To be efficient about providing a 50 ohm impedance to the amplifier the common solution is to use a resonant antenna and a circuit normally referred to as an "antenna tuner", although it really is an impedance matcher. There are thousands of references to antenna tuners and hundreds of different models available to buy.
BUT HERE IS A caution: Radiating a signal from an antenna usually requires a license, and getting that license usually requires demonstrating an amount of knowledge and understanding that you appear to lack. Thus what it appears to be is that it would not be legal.
And if you passed the tset by memorizing the answers you are out of luck.