Hello,
I consider trying a so called flower pot antenna (without flowers or pot).
Till now I could not find info about the optimum height above a flat ground to the lower end, where the common mode choke is. My first guess is 4m agl and 5m of cable from the ht to the choke. The objective is a low takeoff angle.
Does anybody have calculations, simulations or even experimental results to this? TU in advance.
73, Johannes
Hi Johannes,
I have a slim Jim which is similar. From experience about 4m or higher gets best results.
I donât notice much difference between 4 and 6m. Below 4 and I can notice that the performance to hear distant stations can become compromised.
There are times that my mast falls over some of the way and I donât notice 
Tim
Hello Johannes, the originator of the Flowerpot antenna was vk2zoi (sk). Here is a link to his website -
https://vk2zoi.com/flower-pots/
It details everything about it and how it got its name and detail on how to build it.
Geoff vk3sq
Hi Johannes,
For a while I was using a 3m pole, which means the choke was just over 2m from the ground. Iâm not saying itâs optimal, but it worked and I have managed a S2S of roughly 120 miles with it.
Iâve since got myself a 5m pole (still a compromise between size/weight and height), though Iâve not tried it in anger. On a few of my activations, I feel that the extra 2m of height would have helped but Iâve never done a side to side comparison.
Iâm a big fan of the 2m flowerpot. Very happy with mine.
Hi,
Very happy with the flowerpot, I normally use it with the trekking pole (itâs made of carbon).
I havenât tested it more than 1 meter away.
73 José
Hi Johannes
A flower pot is just a vertical dipole without the complication of a feedline at its centre. I modelled the antenna for 145.5 MHz at various heights
| choke height m | gain dBi | peak radiation angle |
|---|---|---|
| 1 | 2.07 | 11 |
| 2 | 3.5 | 8.5 |
| 3 | 4.5 | 7 |
| 4 | 5.1 | 6 |
| 5 | 5.6 | 5 |
| 6 | 5.9 | 4 |
This simulation assumes flat ground !
My model has 1m height in the antenna design, so the chart is the 6M case (5m added)
We want low-angle radiation with VHF from hilltops to reach the chasers, so the rule is - the higher the better. Choose your height according to the size of pole you wish to carry !
By the way I find that a carbon-fibre pole does significantly affect the SWR, but I canât really gauge the effect on the radiation efficiency.
Cheers
Rick
You can hang the vertical antenna for 2m (e.g. Flowerpot) on the top of the mast and operate it together with a horizontal antenna (e.g. Endfed).
To attach the mast, I simply bent a shrink tube.
73 Armin
Hey, thank you all
for the comprehensive information, for the numerous field reports and especially to rick for the simulation results.
My first asumptions are confirmed and I can go on. I will report.
btw to Armin: I will not attach the antenna to the mast. I run a nylon string to the top and down again. So I can change height easily and I can exchange the antenna for another one without bringing the mast down.
btw to rick: I distrust the carbon material too. It is possible to compensate the detuning influence, but we can not compensate the resistive losses it causes.
Thank you all again, 73, Johannes
Hi Johannes
Also donât forget to use a low-loss coax, as you will need maybe 7-8 meters for the feed line at the highest. My actual flowerpot is made from RG-174 to keep it small and lightweight but I feed it with M&P Hyperflex 5 which is ideal.
Rick
Hi Johannes,
I raised a similar discussion a few years ago. My conclusion was that for a vertical antenna on 2m FM the higher that you can get it, the better. The true height above RF ground is unknown on a real summit. The tilting effect of gradients and sharp edges are difficult to model. The higher that you go the more lobes are created and if you are unlucky you may end up with a deep null on your preferred signal path. But overall, the higher the better is my takeaway .
I wouldnât worry about the science too much. Height and ground type can vary from summit to summit. A pointed summit v a rounded one v a plateau will yield different results. I find that the chasers operating conditions make more of a difference than my own.
I certainly noticed holding it vertical helps in marginal conditions.
Thank you for the additional hints. I will take them into account.
Its clear, that the antenna characteristic is not exactly predictable. Lobes and notches will vary with the geometry of the summit and the properties of the ground. By varying the antenna height when listening to a weak signal it could be possible with some luck to readjust the angle of a lobe with good gain.
Johannes
Looking at losses and illuminating the horizon.
Please note that the coil is not a simple choke. It is self resonant at the operating frequency. Itâs job is to isolate the aerial from the feed line and so define the aerial length ( and resonant QRG). Since the end impedance of a dipole is > 2000 Ohms the coil needs special consideration.
The length / diameter ratio will determine if the inter turn capacitance will resonate with the coil inductance at the aerial resonant frequency. I have found that 8.5 turns of RG174 on a 22m dia former is good. My feeder is RG58. Plugs and sockets are SMA types.
To confirm that the coil is doing itâs job, observe the SWR on the feeder whilst holding / not holding the feed line. If it remains unchanged all is good.
Johannes
I would not worry about the lobes. They will be filled in sufficiently by local and distant terrain reflections. If the coil is beyond the reach of a wandering yokel then you have met the EM exposure requirement and main ground influence effects.
Iâve played with different poles - wood, plastic, glass fibre, carbon fibre - and on 10 m could not find any significant difference.
However, although the conductivity of a carbon fibre mast is low it does give a feeling of doing the right thing by avoiding it. Using a pair of walking poles for a mast would be worse.
Operating in a green wet forest will have more effect on your signal than varying the height of the coil from 2 to 6 m for example
In the end Iâm with Fraser. When you are on a mountain you just do what is practical and doesnât involve carrying a lot of heavy stuff.
Just put up the flowerpot and work stations.
73
Ron
VK3AFW
TU David,
I agree.
In the best case the coil is self resonant on qrg. At my 40 m cf dipole I use a tunable trifilar trap aginst cm signals. It works very well and can be tuned by ear just using the receiver.
On vhf most poeple seem to use choke self resonant just below qrg as good enough solution. Tweaking the resonance of the choke can be done later.
Johannes
I think youâll find it is very predictable. What isnât easily measured is the environment, the ground type, shape, slopes etc. Adding that info into the model gives how the antenna will behave in that environment. For most of the time though, simple environment models are used which is why the antenna often behaves differently in situ than when modelled.
Andy,
please excuse my not mastering the english language.
Basically I tried to express just the same content as you did, but I used different words. I do not find a big contradiction. " Lobes and notches will vary with the geometry of the summit and the properties of the ground." was already mentioned by me.
But the ânot exactlyâ seems to be wrong. My fault. I should have written:
The antenna characteristics are predictable, but the prediction can not be very precise due to not sufficiently known influences.
Knowing the summit, we can estimate its influence. But I dont consider this as âvery predictableâ. That means there ist still room to experiment with the reality given in an particular case.
Johannes
Thereâs nothing wrong with your English, Johannes. We all know what you meant (including Andy @MM0FMF), i.e. that unless you know the exact conditions at the summit, e.g. position and type of objects in the near field w.r.t. the antenna, soil/ground type, topology of the summit, etc, etc one cannot know in advance exactly the characteristics of the antenna in situ.
Prediction means modelling or measurement. As someone who created and used models professionally for many years, I know the old maxim âAll models are wrong, but some are useful". In principle, one could go on site with sophisticated measuring instruments and determine (fairly well) the aforementioned physical conditions, do some calculations and then use the antenna. We all know that would be absurd for temporary portable usage, e.g. SOTA, and only a black-belt pedant would have challenged your statement.
Not quite, remember the aerial and the SRC are close coupled and both high Q. One affects the other. I always determine the SRC resonant frequency first using a separate coil WITHOUT FEEDER OR AERIAL. I do this with an ant. analyser and a 1turn search coil. Then I build the aerial using the same coil dimensions and adjust the aerial length to achieve resonance at the QRG. Works every time, both on 2 and 10m
David G0EVV
