Flowerpot antenna trap/choke for 6M

[VK2AVR]

Have been experimenting with a flowerpot antenna, an ingenious way of using a single length of coax as a half wave antenna. Built to spec works fine, but I want a low-profile antenna for 6m and the coax trap/choke is a bit too bulky (50mm diameter). I have been trying to see if it can be replaced by a ferrite choke using a string of beads aka W2DU balun. Using 18 x LF1258 cores from Jaycar this has not worked well, SWR > 2.5:1 while a to-spec one is 1:1.

From reading VK2ZOI's site, the coax choke is tuned to resonate 5-6% below the frequency of interest, so its operation is really a bit more like a trap than a choke. Given it is tuned below the operating frequency I'd expect the impedance to be inductive, good news, this is what a ferrite choke provides. Is the issue that the LF1258 doesn't have the right properties (wrong mix), or does a ferrite bead balun build up impedance too slowly to be worthwhile? (it is 35cm long compared with the coax trap/choke at 6.5cm length).

Cheers

[VK2OMD]

The original choke is best described as an air cored solenoid of coax, and it it designed to be self resonant at a specific frequency.

Whilst the resonator so formed is not a very high Q resonator, it is much higher Q than a resonator formed with a ferrite cored inductor... especially is the material is quite lossy at the frequencies of interest.

Low Q resonators have a broad resonance response compared to a high Q resonator, and Z is resonance is a lot lower than for a high Q resonator.

It is the very high impedance near resonance that is critical to your application.

So, whilst hams love to use so called Ugly Baluns (which are an air cored solendoid of coax which exhibits a narrow resonance at some undescribed frequency) for broadband baluns when they are not as effective as a ferrite cored choke, I expect that the air cored resonator is most suited to your specific application.

Perhaps you should copy and scale the entire design.

If you want to use a magnetic core for the choke, I would try powdered iron as it will give you a higher Q resonator.

Owen

[VK7JG]

Hello Geoff.
And thanks Owen for your thoughts .
I have experienced exactly the same thing .

On VK7RAA 6M repeater we were using a commercial end fed half wave dipole but was suffering from lots of on sit noise and crackling.

I replaced it with a "Flowerpot: and it has never worked better .
I also wanted to eliminate the large coax coil at the bottom of the aerial as it is exposed to severe icing during the Winter being at 4,000 feet on Mt Barrow .

I tried using a single length of LMR400 and fitted a set of five ferrite toroid over the coax only to experience what you have found , they did not appear to be there. They were sold to me by a reputable dealer for use as a sleeve balun on my 6M beam .

I have since build another Flowerpot and made the coil slightly thinner to fit inside the fiberglass radome but compensated the smaller diameter with an extra few turns .

I use old 70Mhz commercial ruggedised radomes they work very well and you can wind your choke around the outside.
Regards.
Joe
VK7JG

[VK2AVR]

Thanks Owen, Joe,

VK2ZOI's site includes scaling for a 6m flowerpot and I have confirmed this works as expected. I suspected the coax trap had a higher impedance. The 6m version calls for a coax choke on 50mm PVC and it's this 50mm diameter choke that is a bit awkward to package. Like Joe I am looking at using this in a repeater installation so it needs to be robust.

Owen, would small powdered iron cores say T50-6 work if threaded onto the coax? Or are multiple turns needed (I thought powdered iron generally needed more turns than a ferrite).

Joe I may end up doing what you suggest.. wind a thinner longer coil on the outside of the radome

[VK2OMD]

Hi Joe, Geoff,

I would think that with that antenna, you expect at the bottom of the nominal radiator you will want to accumulate high charge at points in the RF cycle, and to permit that, you must prevent the charge flowing downwards onto the feed line with some kind of choking device.

There are a plethora of devices that perform this role and loosely labelled baluns.

Is a simple Pawsey stub balun an option, will it be stable in the environment you have.

You might form such a balun by taping another piece of feed line to the existing, postioned so the top end is at the bottom of the nominal radiator, and the low end bonded to the feed line shield. You will need to make this 90° electrical length... ie the differential mode of the two coax shields with jack acting as a two wire line. The applicable velocity factor is NOT that from coax data tables... the reason why this does not work for people so often is they don't understand how it works and apply vf=0.66.

All of the types of treatments that were used effectively on the once popular coaxial dipole are candidates.

BTW, I doubt that making the self resonant air cored coax choke is all that hard.

Owen

[VK2AVR]

A solenoid choke on the inside or outside of the radome is probably preferable but I'll try a pawsey balun just for the experiment.. it is quite a compact idea. I guess my ideas of using something slipped over the coax as a choke are a no-go

[VK2OMD]

A solenoid choke on the inside or outside of the radome is probably preferable but I'll try a pawsey balun just for the experiment.. it is quite a compact idea. I guess my ideas of using something slipped over the coax as a choke are a no-go

Another implementation is a coaxial tube over the cable, top end open at the desired current cut off point, the lower end bonded to the feed line shield to make a quarter wave s/c stub. The problem is with accurately tuning it, and it uses feed line jacket for a stub dielectric, PE is better than PVC if you have the choice.

There is a question about whether these things work properly, and a construction that uses a larger diameter at the top, even a few very short radials may ensure that the stub sets up in the desired mode. Of course if you add short radials, it affects the tuning. (IIRC, King wrote some stuff on this explaining why some types of collinear don't work and some do, I had the quote on my old web site in a discussion about coaxial collinears.)

Owen

[VK2AVR]

pawsey stub doesn't seem to be suitable either. couldn't get it to tune up so i suspect it has similar limitations to the ferrite idea. also, it's far too fiddly to construct.

i'm sure there are 1000 ways to attempt to skin a cat but I was mainly interested in learning about ferrite cored baluns and why my ideas in that area were misplaced. thanks all for the ideas.

[VK2OMD]

pawsey stub doesn't seem to be suitable either. couldn't get it to tune up so i suspect it has similar limitations to the ferrite idea. also, it's far too fiddly to construct.

i'm sure there are 1000 ways to attempt to skin a cat but I was mainly interested in learning about ferrite cored baluns and why my ideas in that area were misplaced. thanks all for the ideas.

How did you try to 'tune' it?

Owen

[VK2OMD]

I checked some measurements I made recently of a pair of RG6 quad shield cables strapped together, and the differential velocity factor of the pair (ie NOT the inside of the coax) was 0.69.

I have seen hams talk about numbers nearer 1, but the bottom line is that you have to measure it, and carefully.

I usually measure it with a GDO. I cut say two pieces of 1m, tape them together, strip a little from one end and wrap some copper wire around both shields and solder. Suspend them in air and loosely couple a GDO to the shorted end, monitoring the GDO freq with a receiver and find the dip freq with the loosest coupling (confirming freq with the rx).

Owen

[VK7HH]

Been reading through some threads and came across this one.

I asked this same question on another thread - http://www.vklogger.com/forum/viewtopic ... 45&t=12790

How do I scale up/down the coils at the bottom of the antenna to suit different frequencies, for instance 52MHz on a 32mm former and know that it is working, especially with such a narrow self resonance. Similar too with a choke for a Moxon, dipole, yagi etc. Best return loss?

I've tried putting the data from VK2ZOI into an Excel spreadsheet with a trend command, however the results do not seem to be correct.

[VK6ZFG]

What about considering an end fed half wave. This would be the same length and could make use of the radome you have.

The half wave can be easily matched using a coax matching stub at the base. In simple terms this is a shorted quaterwave length of coax with the open end centre conductor connected to antenna and the braid to the mast or mounting bracket as a short counterpoise. The feeder is connected at the appropriate impedance point towards the bottom of the quarter wave length of coax. This arrangement provides a DC path to earth for the radiating element.

The J pole uses the same concept. It uses a shorted parallel line quarter wave section with the feed point tapped at the appropriate point along the line.

I use this arrangement for a home and mobile 6M antennas. A 108" CB whip (if you have one) matches up nicely.

I have also replaced the coil (wound on orange electrical PVC conduit!) in the base of a Benelec 10M half wave with a coax matching stub.

[VK6ZFG]

The flower pot type antennas are based on the trapped diople. They are half of a trapped dipole.

The coil at the bottom of the antenna is a trap and with it brings all the issues associated with trapped dipoles. The trap needs to provide a much higher impedance than the already high impedance present at the end of a dipole. It needs to be tuned on the correct frequency to peform that task. A somewhat difficult task. It will also make the antenna frequency sensitive. If VSWR is used as the evaluation criteria this may not be all that evident as this does not take into account as to what is happening to the radiation pattern.

There are better antenna engineering design solutions that use easier, more effective and efficient ways to connect to the antenna radiator.

[VK2AVR]

How do I scale up/down the coils at the bottom of the antenna to suit different frequencies, for instance 52MHz on a 32mm former and know that it is working, especially with such a narrow self resonance. Similar too with a choke for a Moxon, dipole, yagi etc. Best return loss?

Best return loss will do the job especially if you can graph it. IIRC the trap needs to be self-resonant about 5-6% below the operating frequency so when you have reached this point then all the power will be delivered to the antenna and very little will be reflected. You should see a nice well-defined SWR dip. When I was experimenting with ferrite chokes they didn't work and the SWR was all over the shop, not the classic sharp "V" shape it should be. If you don't get a nice "V" shape then it's likely one of the parameters (element length, or trap) is wrong, with the detriment to matching and/or radiation pattern that VK2ZFG mentioned.

If your former is smaller (32mm instead of 50mm) you will need a couple of extra turns. Experiment a bit and see which one gives you the best SWR / return loss.

It will also make the antenna frequency sensitive.

Aren't most antennas frequency sensitive?

If VSWR is used as the evaluation criteria this may not be all that evident as this does not take into account as to what is happening to the radiation pattern.

This is true in the general sense that if you ONLY look at SWR then you can go wrong in all sorts of ways. For instance, I have a magical broadband antenna that is 1:1 SWR from DC to daylight. It's much smaller than a normal antenna although it does get a bit warm when you transmit.

There are better antenna engineering design solutions that use easier, more effective and efficient ways to connect to the antenna radiator.

Perhaps. But what if there is a constraint on maximum antenna length? For example, I want to make a bottom-fed vertical antenna for 50MHz repeater. I don't want radials as they are inconvenient and require the antenna is stood off the tower a fair way. My choices are basically a J pole or a flowerpot. The J pole has the disadvantage of being 3/4 wave long, the flowerpot is a full 1.5 metres shorter at 50MHz. A 3m tall vertical can be a lot more compact and lighter than a 4.5m vertical.

Trapped dipoles are known to work and work well, and a flowerpot will be the same deal. They need to be designed and constructed properly, yes. But it's not that big a deal, if you follow the VK2ZOI plans then it will work. I've made 144MHz and 50MHz versions. We are also going to use a smaller former around 30mm diameter so a couple of extra turns will sort it out.

[VK7HH]

Thank you for your thoughts Geoff.

Best return loss will do the job especially if you can graph it. IIRC the trap needs to be self-resonant about 5-6% below the operating frequency so when you have reached this point then all the power will be delivered to the antenna and very little will be reflected. You should see a nice well-defined SWR dip.

Yep got it. I'm lucky to have a test set capable of all that sort of stuff. I suppose an antenna analyser connected to the antenna on a short mast will also suffice for finding the right area, then connect to the Tracking Gen for a return loss plot for fine tuning later.

If your former is smaller (32mm instead of 50mm) you will need a couple of extra turns. Experiment a bit and see which one gives you the best SWR / return loss.

Probably best to have the whole antenna on the outside of a piece of 32mm conduit to experiment then just cut the bottom coiled bit off to put in a bigger radome. Radomes can have an effect making the antenna appear slightly bit longer.

I have a magical broadband antenna that is 1:1 SWR from DC to daylight. It's much smaller than a normal antenna although it does get a bit warm when you transmit.

At least the antenna's noise level is really low.

But what if there is a constraint on maximum antenna length?

That is just the situation. This will be for a repeater 1250m ASL on Mt.Wellington. Ice, snow, wind. Make it as short as possible too for aesthetic purposes. J-Pole won't last long. Modified Stationmaster won't either. SMD is probably too big. You run out of options quickly!

[VK6ZFG]

In many case the antenna matching system can reduce inherent bandwidth of the radiating element.

The matching stub approach is very simple and I would use it in preference to the tuned coax trap. It is actually quite easy to get going.

My 6M half wave mobile setup is mounted on a commercial 1/2" HF base. It uses either a two section whip made up from ex HF whip parts (so can be carried easily in car) or a 108" CB stainless steel whip . This is fed by RG 58. There is a Tee along the feeder (550mm from the centre of the Tee to the end of the connector plugged into the HF base). For 6M operation a shorted stub (200mm to the centre of the Tee) is plugged into the Tee. Normal HF whips operation is available when the shorted stub is removed. Actual coax lengths will vary with different antenna installations.

To match the antenna I used an AW07A antenna analyser. With this, the VSWR and antenna impedance can be measured. The length to the Tee and the stub is adjusted until the desired result is obained. It does not take long to get the feel of what is needed with the lengths. Note: it is important the the braid of the coax at the antenna is connected to something that will act as a counterpoise. Length not important but presence is.

The coax matching stub does not add any length to the antenna (unlike the J-Pole).

Benelec 10M half wave base arrangement

Benelec 10M half wave with matching coax stub now inside the 30mm dia aluminium tube replacing the original coil wound on PVC electrical conduit previously there. If I was doing this again I would locate the coax matching stub externally. This would give the option of being readily able to use the antenna on other frequencies (eg: quarter wave on 20M).

Additional informatin can be found at:

http://www.arcticpeak.com/antennapages/ ... match.html

[VK6ZFG]

6M coax matching stub