Flowerpot

[toshiba1]

Just built one, quite happy with the results rig is not too far from the antenna and cant get it too high above, seem to get a swr of 1.5 on about 7 watts don't seem too be able to get any lower, if i up power to 18 it does push the swr up to about 2.5 and I get a reflection of about 2w. I'm using black flowfast 34 mm. I reduced the choke from 8 turns to 7. I also from one of the diagrams stripped the sleeve of the lower radiator as in one the diagrams it does show this, but I have seen in some not doing that? Is that beneficial or not? I read from Albert's post that black pipe absorbs RF, by what percentage of power am i losing from that? Gotta say its one of the most pleasurable antennas to build and I'm sure with a little tweak it will work a charm. Love the light weight of it and of-course its suitability to mount easily.

[XXL]

They’re not much good for anything above 100w because of the rg58.

[Albert H]

You might have a problem with the black plastic pipe. If you have an offcut of it, put it in your microwave oven on a saucer, with a cup of water to provide a "load". Run the oven for 30 seconds, and check to see if the plastic tube has got hot. If it has, it's absorbing RF, and isn't any good for your aerial. This might account for your poor SWR. When I've built them, I can get the SWR right down to 1.05 : 1 (on one specific frequency) without much fiddling about. I find that the SWR rises quite rapidly either side of the actual resonance.

[toshiba1]

You might have a problem with the black plastic pipe. If you have an offcut of it, put it in your microwave oven on a saucer, with a cup of water to provide a "load". Run the oven for 30 seconds,

Hi Albert, tried this but forgot about the saucer, just laid it on the glass plate with a cup of water. It did not get warm. I will try the white pipe when I can get round to B&Q. Regarding the stripped outer to the braid, the whole length of the lower radiator, would that effect the SWR? As mentioned the video does not do this but it is shown in one of the diagrams? Albert you mentioned most of your chokes have 6 turns on 32mm. Mine has 7 so perhaps as its 34mm OD maybe I should drop that down to 5, or maybe my inside measurement is off using the calculator the vid links too?

[toshiba1]

They’re not much good for anything above 100w because of the rg58.

Got to admit it does not feel like the same quality you used to get, but maybe thats me rose tinted.

[Albert H]

XXL - I've made these with UR67 and WF103, and run them at up to 600W. RG58 is OK at 150W (that's what a friend of mine has been running recently), but I'd get worried above that kind of power level.

As regards the choke - I've made them at various diameters and with various numbers of turns. The current test aerial I have on my garage roof has 8 turns at ~50mm diameter, and I get a 1.05 : 1 match at 98.4 MHz, rising to ~1.12 : 1 at 97.9 MHz and 98.9 MHz.

[toshiba1]

XXL - I've made these with UR67 and WF103, and run them at up to 600W. RG58 is OK at 150W (that's what a friend of mine has been running recently), but I'd get worried above that kind of power level.

As regards the choke - I've made them at various diameters and with various numbers of turns. The current test aerial I have on my garage roof has 8 turns at ~50mm diameter, and I get a 1.05 : 1 match at 98.4 MHz, rising to ~1.12 : 1 at 97.9 MHz and 98.9 MHz.

Ah so, just read what you posted on the Bazooka Aerial. so took that as gospel. Because its such a simple and cheap to make, I'll try a few different things out and post my findings.

[sinus trouble]

I believe the coils are a Common mode choke!

It separates the antenna section from the feedline coax whilst stopping the coax from radiating!

The important aspect to this choke is the length of the coax! NOT the number of turns!

The choke is actually a 1/2 Wave length of coax @ your resonant frequency!

The coil former diameter is not critical, Larger the former, less turns! (Which helps with lower frequency construction)

I hope this helps

[toshiba1]

I believe the coils are a Common mode choke!

It separates the antenna section from the feedline coax whilst stopping the coax from radiating!

The important aspect to this choke is the length of the coax! NOT the number of turns!

The choke is actually a 1/2 Wave length of coax @ your resonant frequency!

The coil former diameter is not critical, Larger the former, less turns! (Which helps with lower frequency construction)

I hope this helps

Appreciated, I did follow the calculator on the choke measurement twice as I remade it and could not get anywhere near its resonance for the freq. But fear I have a faulty bridge cable short, hopefully its not done any damage.

[toshiba1]

Well unless i am missing something, that online calculator https://nomonsuhendar.blogspot.com/2020 ... lator.html should be taken with a pinch of salt. my measurements seem to be onpoint, unless 0.5 of a mm is crucial to these working. Managed to get a swr of just around 1.3 on a 22mm white flofast, but on a frequency about 100mhz off where i wanted it. lol. It seems the length of the choke is rough at best, sometimes more turns or less. Using RG58. Are there parameters in the calculator i need to change? And is Lamda just the overall length as i did ignore that as it was not in the diagram.

[sinus trouble]

You are not missing something, I get the same outcome!

When i built the Flowerpot for 98Mhz it turned out to be resonant around 101Mhz

This would indicate the dimensions seem to be on the short side? The author speaks of reducing the measurements by a percentage, Which i dont believe is necessary?

I have always used the formula

300 / Freq x 0.935 x 0.5

This will give the total half wave length of the radiator @ your Frequency! Lambda will be double this value!

Im guessing the Author was trying to average out the 'tweaking' factor for simplicity? Who knows??

[toshiba1]

Nice one ST... I will try using that formula. I made another early on this evening with 32mm, I made one slight change i did not strip the outer sleeve for the lower radiator this time. I got a stable swr of just above 1.1 with 4 watts in. And it was giving me 5-6 fwd upped it to 25 and it did not budge much at all. The choke it seems if you take about 180mm / 190 mm away from the calculator measurement it seemed to get good results.

[sinus trouble]

No problem

As with all antenna, they take a bit of experimenting! And there is no need to remove the outer sleeve of the lower radiator!

As you mentioned, you want to use this in the field? The great thing about these antenna are that you dont need the PVC tube!

You can simply roll it up and put it in your bag! Maybe tie some string to the end and sling it up on a tree branch!

[Albert H]

The problem with hanging it from a tree is the foliage. It really attenuates the signal - especially if it's been raining!

[toshiba1]

I had a nice match with a 32flofast pipe, thought i'd transfer that coax to a 20mm pipe, same length on the choke. Does not come any where near a match???

[sinus trouble]

Antennas are never easy to get right!

The strange thing about antenna is that the match seems to alternate at different frequencies?

I tested my Flowerpot using a low power transmitter that could scan up and down the band with the push of a button!

I still believe the measurements given by the online calculator are a bit on the short side?

Hopefully i can order some Coax soon to do more testing soon!

[jvok]

I've used the same calculator and found the measurements came up short too. Although I was tuning it indoors which might be the problem.

I also found the choke doesn't decouple the feedline very well, the swr changes when you touch the coax.

[sinus trouble]

Yes there are so many uncertain aspects surrounding this antenna to say the least!

I dunno whether the velocity factor plays a roll? Or conductor cross section effects the outcome??

As mentioned by Jvok, I still think the dimensions are a bit shy of the mark!

Still! Going with larger dimensions to start with cant be a bad thing? Means headroom for adjustments!

[Albert H]

A ferrite choke beneath the coil also helps to keep RF off the outside of the braid. Velocity factor does matter, and CSA affects the match a bit too. You get differing dimensions for UR67 and RG58 (for example).

[jvok]

My biggest issue with the flowerpot is that its hard to tune full stop, because the coax is hidden inside the conduit. Which means each time you make a change you have to unravel it, cut it down, put it back together then repeat until you get it right.