Flowerpot

[Albert H]

That might be so, but once you've got it right, make careful notes of the dimensions and the materials used, and it will be entirely reproducible.

I have very accurate notes for the 92.4, 92.8, 94, 94.3 and 94.6 MHz ⅞-wave verticals I used to build. These were end-fed (with coax inside their mounting mast), and used shortened radials. When I had stock of the materials, I could make four or five of them in a little over an hour! The longest part of the job was waiting for the Copydex to set (it was used as a sealer and insulator). Once constructed, they could be assembled on site in minutes, and the parts were small enough to carry under a coat. In those days we used to have to smuggle gear on to the sites we used!

[sinus trouble]

Apart from the humble Dipole, I do not think any antenna is easy to tune perfect first time!

All will need some tweaking to suit its final mounting point!

When i build a Flowerpot, I usually secure it to the outside of the PVC tube using insulating tape!

Also a narrow loop in the top radiator can help to adjust the final length?

The choke can be moved up or down without too much trouble too!

Finally! Try not to perform tests indoors! It needs to be a minimum of a metre off the ground and any metallic objects a few metres away!

[sinus trouble]

Another point to mention is that an antenna is a "Reactive" load!

Which means the LC components need to be perfectly resonant! The smallest inbalance can put it out of whack!

Unlike a Dummy load that is "Resistive" Assuming the LC characteristics are close to Zero!

[Albert H]

My "test tower" for Band II aerials is 4m high, and at least 8m in any direction from any other objects around it. It's made from an old folding stepladder and a scaffolding pole, and normally is inside my garage. I put it up in my garden when I need to tune an aerial. It allows me to calibrate aerials with the minimum of hassle. I have a little 1 Watt transmitter that is tunable from 80 - 110 MHz, which has a built-in SWR bridge.

I usually begin with an Antenna Noise Bridge to get the aerial roughly right, then use the little rig for final tweaking.

[EFIALTISFM]

Has anyone tried to add elements on a flowerpot antenna to make it DIRECTIONAL like a YAGI?

[Albert H]

No. It's NOT what the "Flowerpot" is about! The design was for a simple omni-directional aerial.

You can build Yagis quite easily. There are loads of online calculators for them.

The first thing you need to establish is how wide an angle you want to broadcast to - this will determine the number of elements. When I wanted hemispherical coverage (for example), so that I wasn't wasting signal into the largely uninhabited countryside, I used an "H" - just an active radiator and a reflector.

The match impedance of Yagis changes with the number of elements and their positions on the boom. With intelligent design, you can shape the coverage to go just where you want, and get some useful antenna gain as well. As with any simple dipole, you can co-phase multiple aerials and get additional forward radiated power, too.

[EFIALTISFM]

No. It's NOT what the "Flowerpot" is about! The design was for a simple omni-directional aerial.

You can build Yagis quite easily. There are loads of online calculators for them.

Thank you Albert, informative as always.

I'm gathering bits to build a 2 element (maybe a 3 after reading your comment re hemispherical coverage) but can't decide which calc to use as they all give completely different results.

Could you comment or the Rothammel / DL6WU design found here https://www.changpuak.ch/electronics/ya ... _DL6WU.php .

Ta.

[Albert H]

The site you suggest is reasonably OK. However these "calculators" are not perfect: there's no substitute for cutting all the elements slightly too long, and spending (lots of) time trimming them for the best match! Once you've got something that works to your satisfaction, carefully record ALL the dimensions, details of the materials you used, and the SWR that you achieved. You'll then be able to reproduce that antenna design quickly and easily!

https://sites.google.com/view/kn9b/yagi is a pretty accurate calculator - it gives results very close to my measured ones from practical Yagis. The site also has calculators for other types of antenna as well. The one you referred to is reasonably good too, but does point out that the dimensions the programme calculate are "5% too long" - so you're going to have to "cut and try" anyway!

In every case, the calculated dimensions are a good starting point. Remember that everything involved in the construction of an aerial will affect its performance in some way!

[EFIALTISFM]

... there's no substitute for cutting all the elements slightly too long, and spending (lots of) time trimming them for the best match!

Albert, thanks again. I'll extract data from both sites and build the yagi-3. Looking forward to the "trimming" and testing part...

[BlackBeard]

Okay, so I built the flowerpot. Used H155 PE coax.

Diameter of choke: 11.6 cm
Length of choke: 72.88 cm
Turns: 2
Length of upper element: 59 cm
Length of lower element: 59 cm
Velocity factor of coax: 0.8
Connector: PL259
SWR on 101.3 MHz: 1.1:1
Used a NanoVNA-H to measure the SWR.

The aerial was mounted on a roof, with a metre of distance to the rooftop. Used a grey pipe for electrical installations (grey electric conduit).

I chose a quite big diameter for the rf choke because the datsheet says it's minimal bending radius is 60mm. On some sites I read 35mm but I wanted to make sure that the cable doesn't break, it's quite stiff. The aerial itself was attached to the outside of the pipe.

What's weird about it: The measurements I calculated on this site https://nomonsuhendar.blogspot.com/2020 ... lator.html were clearly too long. Also, my choke is much shorter than the calculated 122cm. It seems to me that the length of the choke equals roughly λ/4 instead of λ/2. When I tried the original measurements (x1.03%, made it a little longer), the SWR was completely off. It was 3.5:1 at 90.7 MHz at best. Especially the long choke didn't work out, the SWR lowered when I shortened it.

Does anyone know why this is? I need it to be resonant on 105.4. I was thinking if I shortened the radiators too much but at first it wasn't resonant at all.

[Albert H]

Used a grey pipe for electrical installations (grey electric conduit).

That could be your problem - some coloured PVC pipe is metal-loaded, which can completely mess up your aerial! The way to find out if the pipe is RF-absorbent is to take a short off-cut of it and a cup of cold water. Put both into your microwave oven for about 30 seconds. The water should get warm and the plastic should remain cold. If the plastic warms up, it's no use for aerials!

I always used white plastic pipe, and painted it with acrylic paint when the aerial was finished.

[BlackBeard]

Used a grey pipe for electrical installations (grey electric conduit).

That could be your problem - some coloured PVC pipe is metal-loaded, which can completely mess up your aerial! The way to find out if the pipe is RF-absorbent is to take a short off-cut of it and a cup of cold water. Put both into your microwave oven for about 30 seconds. The water should get warm and the plastic should remain cold. If the plastic warms up, it's no use for aerials!

I always used white plastic pipe, and painted it with acrylic paint when the aerial was finished.

I tried it, the water got warm, the pipe didn't.

Maybe the dielectric or the outer cable sheath of the coax are messing up the aerial? Should I strip the dielectric of the inner off the upper radiator and the outer cable sheath off the lower radiator?

It's also not that the antenna is not working, the measurements just differ significantly from what I expected. Also, when I touch the NanoVna, the SWR curve shifts to the right (higher freqs) which could (?) be a sign of not enough decoupling happening? Maybe I'm mistaken.

[Albert H]

The dielectric will be electrically inert, so leave it on the upper conductor for strength.

There's the original Flowerpot design on https://vk2zoi.com/articles/half-wave-flower-pot/ which gives dimensions for 144 MHz and 50 MHz. There's also designs for variants of the basic half-wave, including a ⅝-wave version and "twin" versions with much more gain.

Way back, I found the dimensions for 50 MHz, 70 MHz and 144 MHz, and drew graphs for the various dimensions. It was then easy enough to interpolate the various frequencies in Band II as well. When I've built them, the dimensions have varied slightly (according to the materials I used) and I most usually made them with UR67 cable (I had loads of offcuts of it). Changing to RG58 changed the dimensions by about 4% as I recall.

The real beauty of this aerial is that it works well, and isn't obvious. We used to paint them (with acrylic paint) a pale grey which would be virtually invisible against a northern European sky for most of the time. It's also quite a lightweight aerial, so tall supporting masts didn't have to be especially strong.

[BlackBeard]

Way back, I found the dimensions for 50 MHz, 70 MHz and 144 MHz, and drew graphs for the various dimensions. It was then easy enough to interpolate the various frequencies in Band II as well. When I've built them, the dimensions have varied slightly (according to the materials I used) and I most usually made them with UR67 cable (I had loads of offcuts of it). Changing to RG58 changed the dimensions by about 4% as I recall.

The real beauty of this aerial is that it works well, and isn't obvious. We used to paint them (with acrylic paint) a pale grey which would be virtually invisible against a northern European sky for most of the time. It's also quite a lightweight aerial, so tall supporting masts didn't have to be especially strong.

That's actually why I wanted to build the aerial! It's cheap, compact (can be put out of almost any small roof hatch), easy to build and lightweight which doesn't require a complicated installation and a super strong mast. Also, if the plastic pipe would break or fly away during a storm, probably no one would be harmed because of the light weight.

Concerning the dimensions, the 4% difference from RG58 to UR67 you mentioned are pretty much equal to what I noticed. I just have to have a look again at the rf choke, maybe I'll try a smaller diameter (although that shouldn't make a difference as you pointed out). Is there possibly a minimun amount of turns (like 4)? I only have 2 turns. I also never saw someone making a choke with only 2 turns. On the other hand I read, that the more turns the choke has, the more lossy it gets.

[Albert H]

My Flowerpots had 5, 6, or even 7 turns in the choke. The loss through the choke is minute. What you're trying to do is prevent RF running down the outside of the feeder braid. I also would clamp ferrite filter blocks to the outside of the feeder below the choke - this helped at lower frequencies (like 50 MHz), but had less effect at Band !!.

[Albert H]

I've also made the ⅝-wave version of this, and got ~1.9 dBd gain in return for a bit of constructional hassle. I've also made the double-⅝ version, and measured the gain at ~5.4 dBd, which is a really useful return.

The radiation pattern of this aerial is roughly doughnut-shaped, and the low angle radiation is minimised, so that you put much less signal into the immediately surrounding neighbourhood. In broadcast terms, the ⅝-wave version is at least as good as the old NRG ½-wave vertical, and is easier and cheaper to build. It's also not quite as narrowband as the NRG aerial, so I always found it easier to tune.

[radionortheast]

You can get fm halo aerials meant for recieiving, bend them out, their much lighter than the alliumium tubing you can get from ebay, you can make your aerial from that, use thicker allumimium to join them together, screw them together to make a vertical, a wire for the matching section held apart with some abs plastic, i’ve made end fed dipoles this way it worked well, would withstand been outside. If you’ve got the aerial in sections it will make it easier to get through your roof if you’ve got clearence problems.

[BlackBeard]

I've also made the ⅝-wave version of this, and got ~1.9 dBd gain in return for a bit of constructional hassle. I've also made the double-⅝ version, and measured the gain at ~5.4 dBd, which is a really useful return.

The radiation pattern of this aerial is roughly doughnut-shaped, and the low angle radiation is minimised, so that you put much less signal into the immediately surrounding neighbourhood. In broadcast terms, the ⅝-wave version is at least as good as the old NRG ½-wave vertical, and is easier and cheaper to build. It's also not quite as narrowband as the NRG aerial, so I always found it easier to tune.

Thanks Albert & radionortheast I finally managed to tune it correctly. I remade the choke. My new choke now has 6 turns on a diameter of 7.6 cm. Works better. Do you mean this version: https://vk2zoi.com/articles/double-five ... lower-pot/ by double ⅝?

Originally, I wanted to build the Moxon but the double ⅝ seems to be a little easier to build and probably less prone to wind. And the double ⅝ seems to give more gain than the Moxon, doesn't it?

As a compromise, I first tried out the simple flowerpot. If it would be a success, I'd continue with the double ⅝.

[Albert H]

That's exactly the one.

The Moxon can be a good aerial, but requires a lot of work to build.

[BlackBeard]

Is it possible that the lower radiator is longer than the upper? My lower radiator is now 54cm and my upper 52cm (H122). The SWR is 1.4:1 at 105.4.