Hello all,
I have been looking for a dual band homebrew antenna with some gain for sometime and came across the Dual band Collinear flower pot antenna, it will just about fit in my loft ( I favour stealth operation) and with the new coax I have ordered and the fact I’m using a 2 meter dipole for 70 cms it should offer 10db+ of gain on 70cms and somewhere near 4 or 5 db on 2 meters ( I’m using ropy RG58 at the moment 
) I wondered if any of you have tried the design and what were the results ?
Yes.
I tried making my 2m flowerpot dual band with 70cm by fitting foil wrapped sleeve over the centre section.
I found that it would now resonate for 70cm but the swr was well off for 2m.
I tinkered with it for a while but got no joy and in the end just made a separate flowerpot for 70cm.
I mostly do sota and not much 70cm work with my ft818, but will usually try a 70cm call from my handheld whilst packing up the main antenna. Unless of course Viki M6BWA is around !
Andy
MM7MOX
and what were the results ? I have built a few of the standard dual band ones in the past with good results but thought if I’m going to the hassle I may as well try and get as much gain as possible !
I live in a valley and have an 850m mountain to the west, hills to the north and a wooded hillock directly south. Gain isn’t the issue here.
It works though. I didn’t measure/model the gain. It’s 5m up and survived Storm XXXX*
*insert storm name of choice
I have always found it the other way, I would start with the standard 2 meter one get that working with low SWR and then add the phasing sleeve. This always seemed to send the SWR on the top end of 70cms up to just within limits but it did work much better than using a standard 2 meter flower pot. It seems quite difficult to find home brew plans for something that will work well on both bands rather than just resonate on 70cm but with poor results. I did think about going 70cms only but as the band is so dead here with very little traffic I would have to commandeer a 70cm repeater in order to link it to a 2 meter one !
My location is not as bad as that but also not ideal with rising ground to the East, large Hills to the West although there is a HUBnet repeater on 70cm and a 2 meter repeater on them which is slightly shielded to me by a closer smaller hill ,to the south I have the entirety of the Wyre Forest and to the North slightly rising ground near by.I do get into a few repeaters with a poor signal on 70cms but am hoping to improve that with the antenna and better coax. Thanks for your info on the collinear . I’m waiting for delivery of the coax and will have to route it from the ground floor to attic but intend to build the antenna in the next couple of days.
Maybe you could post the link to the design you are considering so everyone can see which of the many variations you are talking about?
This one https://vk2zoi.com/articles/dual-band-high-gain-flower-pot/ tried it today and in no way could I get it to match on both bands, I’m either going to have to give up and go one band or the other or revert to the simple dual band flowerpot
“Dimensions shown are for a 1st build prototype from a concept drawing and have yet to be refined.”
Say no more.
I thought it worth a try and tried altering the sizes and phasing sleeve position but of course it didn’t work ,my next plan is to go with a 2 meter 5/8th wave ground plane and either a 70 cm 1/4 wave or 5/8th wave fed at the same point other than that I’m going to have to give up on one of the bands and concentrate on the other. Tomorrow I’m going to test a dedicated 70 cms dipole as it’s easy to do and at a later date the above experiment. The coax should give me approx 3 to 4 db improvement. At present I can do better on a handheld with a short antenna on 70cms by moving location a little and circumventing the coax !
For an attic antenna, I would suggest getting a sheet of corflute and making your antenna with copper/aluminium tape.
It is both cheap, and very fast and easy to make and adjust. The 70cm sleeve choke, which is sensitive to both spacing and length, is especially easy to adjust.
You can make any type of antenna directional or not, here’s a 2m/70cm moxon
Foil antennas can easily have very wide elements for better bandwidth if you want.
This is far and away the best, cheapest way to make VHF/UHF antennas, except for that pesky wind thing.
It’s a widely misunderstood fact that an antenna is not resonant on 3 times its fundamental frequency.
The length that resonates on say, 145 is definitely not resonant on 3 times that, or 435. Its resonant frequency is more like 3.5% higher than 435, or 13 mhz higher. The reason is that the resonant length on 145 has been adjusted for end effect, so is say, 5% shorter than a physical length would suggest. Multiplying the fundamental resonant frequency by 3 gives an incorrect answer as the antenna is a physical length for 3.5% higher than 145, which is about 152. A diagram would help illustrate it, but for now, accept that at 3 times the frequency, the antenna is too short for 3rd harmonic resonance at 435.
This effect is also found on the 3:1 ratio of the 40m and 15m band, the 40m antenna resonant on 7.1 is not also resonant on 21.3, it turns out to be resonant on or above 22 MHz. Add a couple of short wires to it and it can be resonated on 21.3 or wherever you want it, without making much difference to the fundamental resonance.
The problem lies in the assumption that end effect applies to each half wave of the antenna, when in reality it only applies on its ends. And therefore must be calculated at 5% of a half wave at the higher frequency, rather than at the fundamental. There are only 2 ends of an antenna, not 6.
The outcome is that the dual band antenna is a compromise and will work perfectly on one band but not both at a given length. Make it a bit longer for 2m in order to get acceptable performance on 70cm, when considering swr. It will probably not affect physical performance, gain, dx worked etc at all but the mighty SWR meter tells a different story, so worries us.
73 Andrew VK1DA
Yup, and fixed easily for a link dipole.
I really struggled with making a 70cm flowerpot - perhaps the coax I was using was too thick & stiff.
Keen to know what you used- want to have another stab at it. 
I used RG-58 for the 2m flowerpot but found a piece of RG-316 in the bin and used that for the 70cm one, I guess RG-174 would also work if you account for any change in velocity factor.
Hope it works out for you.
I also made a flowerpot for 6m as well which has worked well locally but I haven’t managed any DX with it yet. Used RG-58 for that one as well.
Andy
MM7MOX
The coax is important. Dimensions, coil details will vary for different brands of nominally the same coax. i.e. one brand of RG-58 may have quite different properties to another. They may be 50Ohm but the VF, capacitance/length, inductance and physically sizes can vary. That can be enough to make the published dimensions “wrong” when you try them with different coax. David @G0EVV has written on here about how to check the coil is correct and it’s well worth reading.
It’s worth stressing that the higher you go in frequency the more critical dimensional accuracy becomes. So if design ‘X’ dimensions are critical to the nearest 3mm on 2m, then it will be 1mm on 70cm. Also some designs are affected by local objects. In use these antennas may be a few m above ground and such positioning can be hard to achieve when trying the antenna indoors.
I built a 2m flowerpot to the design details provided by David G0EVV. Works absolutely fine, Almost 1:1 SWR and I’ve used it on a 5m pole, stuffed in a bush and also in my roofspace. I wouldn’t worry too much about environment as long as it isn’t near metal flues, water tanks, etc.
I think that was the issue - RG58 being too fat to wind around my choice of tube (for the choke)… the tube was probably too wide in the first place.
My coax can be used for the 2m flowerpot, or the EFHW. If I add a 70cm end to it, I have all bases covered.
It looks nicely made. I am surprised that local stuff doesn’t affect Gerald’s. After I built mine I cover the choke windings with a nice piece of heatshrink (the glue lined stuff that is not really glue lined but cross-linked differently). The SWR went to hell in a handcart so I had to remove it and it looks crufty now but the match went back to before. 
