88-108MHz Antenna DIY88 - 108 MHz Antenna DIY Easy to build, simple parts and very high performance.
Here you will find information how to build a high-performance antenna for 88-108 MHz band.
You can of course tune it for other frequencies as you wish.
There are three important words you will hear here.
I will explaint them for you, and I will make sure you will have no problem to follow me.
The words are: dipole, impedance and balun.
You can not escape from them if you want a high performance antenna, but it will be worth the reading effort.
At the end of this page you will be hooked and planning for your own antenna.
Features:
88 - 108 MHz range
Only coax cable as material
High power tolerance (100W)
SWR 1.0 to 1.2
All contribution to this page are most welcome!
Background
This project explain how I built an antenna for 88 - 108 MHz.
I want the antenna to have the best performance and still be easy to build.
To build a good antenna you need to understand three words (dipole, impedance, balun) and how they affect the performance of the antenna.
I will explain them and I will share my own measurements with values and diagrams.
Let's start the journey together and at the end you will know all you need to build your own antenna, great!
Hardware and schematic Dipole
First we will look at a very simple antenna called dipole. (Look at the picture below.)
It is basically two rods, one going up from the center and the other going down.
The length of the two rods decides the resonance frequency of the dipole antenna (working frequency), more about that later.
So how does the radiation pattern look from this antenna, well look at the picture below).
You will see a sphere around the two rods like a donut, and the red area show the strength of the signal, as you can see the antenna
has most strength at the outer edge of the donut.
We do not want our rf signal to go high up in the sky (we are not communicating with airplanes or satellites),
we want it to go straigt forward and in a circle around us.
Now, when we have chosen an antenna type, are we done?
No, we have to deal with next word Impedance!
The impedance of the dipole is (73 + j42.5), well that is no good, and what does that actually mean?
It means that the resistive part is 73 Ohm and there is an inductive part of j42.5 which would be 67 nH at 100MHz. This is total useless for us, right!
There is a point (in frequency) where this inductive part is gone and the antenna is only resistive. The resistive part is then 70 Ohm.
Still not good!, we want 50 Ohm (Often a standard when it comes to radio), not 70.
Alright, there is a way to achieve 50 impedance by moving the rods so they point forward in a V-shape. (See foto below)
Here is a link to calculate antenna rod length: https://www.wireantennas.co.uk/dipole-calculator
I got the best performance at 100 MHz when the rods were 70 cm. The online calculator for dipole gives 71.32 cm.
My V-antenna has 98% length of the online calculator dipole.( 0.98*71.32 = 70 cm)
(Be kind to me, this is a prototype built of cardboard, and 16 mm VP tubes for electrical installation. The hot glue-gun did a great job.)
As the V-angle gets smaller, the impedance goes down from the normal 68.9 ohms and passes through 50 ohms to give a match with coaxial cable.
This 50 ohm point occurs with the V-angle around 120°. Great!
The drawing also show the dimensions of the antenna. I use two RG58 coax cables as rods. The length of each rod is 70 cm. The reason I use RG58 is because it is
cheap and the outer shield has a diameter of 5 mm which gives a wider rod. I want to make clear that I use the outer shield (braid) as conducting wire "rod".
I put the RG58 coax cable inside the VP tubes to keep it in place.
You can of course any wire or even tubing. Some use copper tube or equal. I do like the RG58 because it is cheap and simple to find. A wider rod diameter gives a slighter wider bandwidth as well.
The angle between the rods was 110° in this construction, it gave me the best results, which I will show you a bit down in the text.
Now we have managed to handle two of the 3 words, what about the final one (balun)?
As we said in the beginning we use two rods. Since we have two rods, the rf signal over them should be opposite in polarity. Often we call this type of antenna balanced.
All the magic happens between the two rods in a dipole and V-shaped dipole.
The length of the rods will set the resonance frequency of the antenna. This means that we do NOT want any other wires to affect this relationship.
Imagine now that we connect a 50 ohm feeding coax cable to the antenna. The inner wire goes to the top rod and the outer shield "braid" goes to the lower rod.
Our V-shaped dipole has now been affected by the feeding coax cable, and specially the bottom rod which is connected directly to the braid of the coax. The feeding coax will act as ground plane.
The antenna has 50 ohm impedance and the coax cable is 50 ohm, but the feeding coax will become a part of the antenna and affect it, specially impedance.
The perfect antenna will be very sensitive how you place the feeding coax cable, and this will affect the impedance of the antenna.
In worst case, the antenna will be detuned and you have poor performance.
The reason we get this problem is that the feeding coax is un-balanced where the outer shield "braid" is connected to ground system and minus pole of power supply.
The inner wire is connected to the transmitter/receiver. The braid will act as ground plance and affect the antenna system.
So how do we "disconnect" the feeding coax from the antenna so it won't affect it?
It is here the balun comes in.
The balun will connect the un-balanced feeding coax cable to the balanced antenna so rf power can pass through and still prevent feeding coax cable to affect antenna performance. Great!
You can now place your antenna at a good spot and the feeding coax cable will not affect the antenna performance in a bad way anymore.
So what does a balun look like?
You can find many solutions out there and some are more complex than other. You will find ferrite cores, twisted coax cables and other just skip it.
Now, there is a simple balun you can build out of coax cable itself and it is easy and work very good.
Picture below will explain how to do.
Here is a link to the original page where I have borrowed the drawing. I recommend to read his work, great job, lot of knowledge there. https://www.hamradio.me/antennas/coax-velocity-factor.html
What you do is that you build a balun with two coax cables and connect them to your feeding coax. One cable should be 1/4 wavelength and the other 3/4 wavelength.
In the picture above, you find your feeding coax at the bottom, blue arrow. You connect the two wires to that point. The 3/4 wavelength is longer and you can make a small loop of it.
At the other end, (top of the picture) you solder the outer shields "braids" together.
The two inner wires will then be the balanced output to your antenna.
Here is a good explanation on youtube about the subject : Transmission-Line Baluns
Picture below show the connection between the balun and the antenna rods. You can see how the inner wire from the balun is connected to the "Resonant Balanced Antenna Load", which
is the antenna rod going up and the other going down.
How to build the Balun
Let's start with the balun and test it by itself.
I will use a RG316 coax cable as feeding cable. RG316 is simple to find and not expensive.
First we need to calculate how long 1/4 wavelength is when we use RG316. We will design it to work at a frequency of 100 MHz.
First I need to check the velocity factor of RG316 which is 0.695 according to datasheet
Here you can find a great online calculator to help you out: https://k7mem.com/Ant_Freq_Wavelength.html
I enter 100 MHz and my own velocity factor of 0.695. I check "Yes" to Apply velocity factor.
At the bottom you will now see that a 1/4 wavelength with velocity factor is 0.521 m and the 3/4 wavelength is 1.563m
I solder the two coax cables to my main feeding line as the picture showed. To be able to measure the performance I add a perfect 50 ohm dummy load at the balanced output, instead of antenna rods.
In this way I can measure the performance of the balun itself. I connect it to my VNA and sweep from 50 MHz to 150 MHz.
Below the picture I will explain what we see.
Here we see a graph over the return loss of the balun. My VNA sends out a rf signal from 50 MHz to 150MHz and measure how much power that goes out and how much power that returns as reflection.
Reflection happens when there is mismatch in impedance. To the left is S11 which is the ration how much power that are reflected compared to outgoing power.
An important part is the scale which is logarithmic. If you have total (100%) reflection, you will have 0dB. At -10 dB you have about 10% reflection of power. At -15dB you have 3% reflection.
Look at the bottom of the page and you will se a table showing the relationship between dB, SWR and % reflected power.
Remember, we want as little reflection as possible. It only happens when impedance is 50 ohm.
In the diagram you can see we have a bottom around 102 MHz and we have about -32 dB which is less than 0.1% reflection of power. It means we have perfect match at that frequency.
Let's go up a bit to -20 dB and here we have about 1% reflection of power, which is nothing in practical life. The rest 99% of the power goes out from the balun to the antenna.
We have a band from 97 MHz to 106 MHz where the balun has extremely good performance, specially at 102 MHz.
The S11 can be in dB, but often it is written as VSWR and -20dB is equal to 1.22 in VSWR, again very good value.
Here is a pdf where you can see a conversion table about dB to VSWR and formula : Return Loss to VSWR Conversion Table
From this we can say we have a great balun and our calculation of coax length worked well.
Let's remove our 50 ohm dummy load and connect the two antenna rods and measure the complete system with our VNA, shall we.
Now we have all three parts, let's make a real thing and investigate the performance.
V-shaped Dipole
Here we see a graph over the return loss of the total antenna system. My VNA sends out a rf signal from 80 MHz to 120MHz and measure how much power that goes out and how much power that returns as reflection.
Reflection happens when there is mismatch in impedance. To the left is S11 which is the ration how much power that are reflected compared to outgoing power.
In the diagram you can see we have a bottom around 100 MHz and we have about -36 dB which is less than 0.04% reflection of power. It means we have perfect match.
Let's go up a bit to -20 dB and here we have about 1% reflection of power, which is nothing in practical life. The rest 99% of the power goes out from the balun to the antenna.
We have a band from 99 MHz to 101 MHz where the balun has extremely good performance, specially at 100 MHz.
At -15 dB we have 3% reflection and the frequency band is 98-103MHz, again very good.
Why is there a bump at 106 MHz, well it took me a while to understand it, and then I remembered we have a strong commercial radio channel at that frequency.
This signal interfere with the VNA instrument during the antenna measurement.
Why is the antenna bandwidth so sharp, how can I make an antenna that match the entire band of 88-108 MHz?
This is the typical performance of a dipole, one can increase the bandwidth by making the two rods wide in diameter.
Still, it is very difficult to build a dipole for entire band 88-108 MHz with high performance.
There is always a tradeoff. In my example I designed the antenna for 100 Mhz and I will use it only in that range.
If you will be using it at 88 MHz, it is much better to design it for that frequency.
Tabel showing relation of return loss in dB, SWR and % power loss
Final word
I hope you have enjoyed reading about this antenna rf project.
Hopefully you have found new inspiration for your own projects. If I have been unclear or made any misstake, please contact me.
If you wish to offer me a cup of coffee I will be very happy, hit the button below.
88 - 108 MHz Antenna DIY
