Lets look at this question in two ways:
- The first would be if the question is aimed at local QSO’s (as many CBers use the vertical for “local” use)
- And secondly; When we adress this question for “DX” (shooting skip).
But…., Lets start with something that is commonly misinterpretated:
Im sure you have searched the internet for information, which is a good thing !
But not everything is “true”…
Every now and then you will see the plot below when a manufacturer or “expert”
tries to explain that the 5/8 wave antenna is best :
Somthing similair like this:
THIS IS ONE OF THE MOST MISLEADING PLOTS SEEN FOR VERTICALS.
The plot is true, however it is made above “perfect ground”.
That perfect ground does not exsist.
A salty sea doesnt come close.
It is MISLEADING and DOES NOT reflect the real world.
Glad to have that out of our way, lets move on 🙂
HOW HIGH SHOULD I PLACE MY ANTENNA FOR BEST LOCAL QSO’s ?
Lets start of with a few simple “traditional” guidelines:
A rule of thumb often used on VHF is : what the antenna can see is what we can work.
That (radio) line of sight can be calculated with:
d=4,12 √h
d= distance between transmitter and receiver in (km).
h = antenna height (m)
Example : for a vertical antenna at 20 meter heigth we find : 18,4 Km. this is “direct line of sight”.
If the receiver has his antenna also at 20 meters, the distance is doubled.
But we often can communicate much futher especially if we use a bit of power.
That is because we still can take advantage of the “surface wave” on 27MHz.
That surface or groundwave is very effective on Medium Wave bands but becomes less effective higher in frequency.
A rule of thumb to calculate the distance is:
d= 200 / √ f
d= distance (km)
f= frequency in MHz.
Example: 200/ √ 27 = 38,5 km
The above obviously highly depend on power and modes QRM etc. It is no problem to work stations upto 100Km away on FM with a bit of power and two high placed antennas, but it is a starting point.
There will be influence or perhaps negative influence due to citys/mountains/ground etc.
A usefull tool to get a rough idea of your “FM local coverage” can be found online :
Just sign up and put in your location power antenna gain and you can get the following graph:
Please keep in mind the values are averages and can certainly not be used as “facts”.. Simply cause the database doesnt contain all the buildings bridges and local ground conductivity etc.
But it is very usefull, you can find out with increasing your power or antenna height the range will become larger.
(And ps it is fun to look up your competition and compare ranges.)
So, although we cant give a definate number in kilometers….we have learned that besides power, antenna heigtht is one of the key aspects to enhance your coverage area.
Now let us look at some of the antennas often used:
(below data is from the sigma 4 article)
A dipole, ground plane, a 5/8 wave and the Sigma 4 (or J-pole).
I have used these antennas without any loss or introduced other negative effects like common mode currents. etc. In a real situation results can be worse ! As most “verticals” have negative influence
What happens if we place those antenna on a fixed mast and start to calculate the expected strength at a receiver 30km away from the transmitter (receiving antenna 10m high).
We notice that for an average user the Sigma IV or other J-Pole equivalent is perfectly capable of “showing” gain up to several dB ‘s.
(Again: we have a “fixed” mast length, between 1,5 and 24,5 meters and are watching the strength of the signal
change at a distant location for various antennas on that mast)
We also notice that the difference will become less with added height.
(Please bare in mind -74 (dB) is stronger then -76 (dB) …just to have it said.)
What will happen if we place all antenna TIP heights equal ( some antennas need a longer mast)
We can see the Sigma isnt the best anymore but all others are more or less equal.
THE KEY IS ….ANTENNA TIP HEIGHT.
CONCLUSION HOW HIGH SHOULD I PLACE MY ANTENNA FOR “LOCAL” ?
THE ANSWER IS : AS HIGH AS POSSIBLE ! …ANTENNA TIP HEIGHT RULES.
There is NO “ideal” height except higher is better…simply cause the possible different far field take-off angles are of less interest.
There is one “CATCH”, make sure your antenna performs without negative side effects. Use radials, use RF-chokes and limit material losses.
HOW HIGH SHOULD I PLACE MY VERTICAL FOR DX ?
One of the often heard so called “rules” is the antenna needs to be one wavelength above ground. (11m / 36 feet).Again, im afraid this isnt true… but ill try to explain where that idea is comming from.
Let us look again at that “perfect ground” (which does not exsist, but was often used in the old days for “easy” calcualtion).
Below you will find a GIF with a traditional groundplane antenna ( 1/4 wave and 4 radials) above a perfect ground.
Now, i would like you to pay attention to the plot at 5,5m (1/2 wave) and 8,25m (3/4 wave) and finally that notorious 11m (36 feet / one wavelength.)
What we can see is the second lob comes down when heigth increases. That real high angle in green (totally useless for DXing) comes down …and becomes “a usable angle ” as we reach that full wave length (in black).
That is the idea behind that often heard 36 feet / one wavelength statement.
BUT WE DO NOT HAVE A PERFECT EARTH ! …
Please forget about that “rule” it is based on old very old traditional calculations, and has nothing to do with our real situation.
The 1/4 Wave Ground Plane above REAL EARTH
First lets have a look at a quater wave vertical with 4x 1/4 wave radials, no mast, no losses.
In the GIF you can see the antenna above flat different grounds. Primairy is “average ground” Blue is very good ground (near or almost in sea) and Green for very poor ground (industrial).
Lets look at a few individual:
With the groundplane just 1 meter above ground, we can see clearly the ground has a hugh effect. A quater wave vertical above very good ground its beats the one above average ground with 3 dB and almost 6dB compared to a very industrial enviroment. (for a 9 degree elevation angle).
If we place that ground plane antenna at a halve wavelength (5,5 meter) we can find:
The advantage has become less at a 9 degree elevation angle the difference has dropped to a maximium of 1,8dB in favor of the “very good ground” situation.
But that advantage of the very good ground becomes a disadvantage if we go higher.
Below the plot at 11 meters heigth.
The antenna above very poor ground, now has a 3dB advantage compared to very good ground. (at 9 degree elevation angle)
Lets do the same with a vertical dipole.
Click on the GIF below to see the antenna pattern change along with heigth over various grounds.
(black is average ground, blue is very good ground and green is very poor ground.) Again there is no mast, and the antenna itself is “perfect” no losses from matching systems or common mode currents etc).
If we look at these plots individual :
Again we can see the very good ground provides an advantage amost 2dB from very poor ground to average ground and an additonal 3 dB above very good ground.
With the antenna situated a 5,5 meter above ground we can see again “things’ become more equal for low angles, though the dipole above very good ground has a big dip which should be considered as a disadvantage as most stations we normally work arrive at that “dip”.
If we place the antenna higher for example the 11 meters above we can clearly see good ground has become a disadvantage and the poor ground is the winner !
We could show you the plots for a 5/8 wave or Astroplane or Sigma 4…but the essence remains the same.
CONCLUSION :
Always make sure you have the antenna free from obstacles in any situation. Then without a doubt you will work DX and “get out”. It actually is difficult to put up a antenna that wont make contacts.
There are however guidelines to achieve maximum performance:
If you have a vertical antenna above very good ground, near a salty sea on a portable island expedition or perhaps from your favourite DX spot..a very moisty area..or fresh water spot:
It is wise to leave the antenna near the ground. It actually has a negative effect if you would raise the antenna higher then 5..6 meters above that good ground for DXing.
On the other hand;
If you are in an “forest”, rocky land or worse “heavy city / industrial” it woud be more bennificial if you do the opposite and place that vertical preferbly higher then a 1/2 wave for “low angle” Dxing.
(Still under construction :
-needs to be added: examples with mast and explination why vy good ground can be a disadvantage)