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 Feedpoint 
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Hi just an enquiry . Can a dipole be added to the same feedpoint as a Yagi.For example, if i had a triband yagi (20-15-10m) can i add a 17m inverted dipole to the same feedpoint without it interacting with the yagi and if so how far can i put the feedpoint from dipole to the feedpoint to the yagi. Also when a dipole is up about 10m what would be best , inverted v or v or does it make a big diffrence?

Thanks Drew


Tue Jan 04, 2011 2:11 pm
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 Feedpoint 
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Basically, no especially taking into account you are wanting minimal or zero interaction. The idea of having a shared feedpoint is the antenna feedpoints are at the same position, i.e fan dipole or cobwebb or multiband hexbeam/moxon. Or are you talking about a shared mounting point such as a single pole with both antennas on but with seperate co-ax feeding each antenna?

Your dipole will interact in most average gardens but to how much....

As for flat-top vs inverted V, the inverted V will be more omnidirectional but if you can use a rotator then go for the flat-top.

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Tue Jan 04, 2011 4:37 pm
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 Feedpoint 
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Hi Conor and ta for help.I was thinking more of a single coax and the way a Hexbeam was fed with a little coax feed between each element or does that little bit of coax feed between each element become part of the antenna? I was asking witch way up and the diffrence between the two i.e. the dipole( inverted v) or as a( V) ? I was thinking inverted V just under the yagi with single coax and feeding them both like the Hexbeam does between elements.I hope this makes better sense.

Ta Drew
M0GVZ wrote:
Basically, no especially taking into account you are wanting minimal or zero interaction. The idea of having a shared feedpoint is the antenna feedpoints are at the same position, i.e fan dipole or cobwebb or multiband hexbeam/moxon. Or are you talking about a shared mounting point such as a single pole with both antennas on but with seperate co-ax feeding each antenna?

Your dipole will interact in most average gardens but to how much....

As for flat-top vs inverted V, the inverted V will be more omnidirectional but if you can use a rotator then go for the flat-top.


Tue Jan 04, 2011 5:16 pm
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 Feedpoint 
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If you are talking about a truly common feedpoint - yes it's possible, although you would see some deterioration in the VSWR. For example, a 17m dipole has an impedance of about 42-j317Ω on 20m. So if the beam on its own was a perfect 50Ω on 20m, we would now be placing 42-j317Ω in parallel with it which would result in a worsening of the 1:1 VSWR to about 1.2:1. Any distance between two feedpoints on the same coax complicates things further because of the impedance transformations which can then happen.

I'm not sure how you would achieve a truly common feedpoint if the beam is rotatable.

If DX is your thing, erecting the dipole as a flat-top will do better than an Inverted-V at low take-off angles in the broadside direction. But as Conor suggests, to get full benefit it would be useful to be able to rotate the dipole. Any chance of building the dipole from tubing, or even fishing poles supporting wire, and attaching it to the same stb mast as the beam?

Steve G3TXQ


Tue Jan 04, 2011 5:21 pm
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Our postings "overlapped". The lengths of coax between feedpoints on the Hexbeam are fine - you wouldn't want to go much longer. They don't become part of the radiating structure, but as I said earlier they do introduce impedance transformations.

Picture the situation where you had a 17m dipole about 8ft below the beam on a common piece of coax. When operating on 17m you have an 8ft coax stub projecting beyond the feedpoint which is almost Open Circuit because it sees only the very high "off-tune" impedances of the beam. That 8ft of coax acts as a 1/4 wave transformer and results in some very low impedances across the 17m dipole feedpoint. The same mechanism is the reason we always connect the main feedline on a Hexbeam to the 20m position rather than the 10m position.

73,
Steve G3TXQ

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Tue Jan 04, 2011 5:30 pm
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 Feedpoint 
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Thanks Steve all understood and gives me lots to ponder.

Ta Drew


Tue Jan 04, 2011 8:14 pm
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 Feedpoint 
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Cheers Kev,but i live on the ground floor of a block of flats and i have to hire a ladder etc for access .I got the ok for a mini beam that i have up and i am easing them in with that so i like to keep everything minimal and everyone happy.It has been up a year without a complaint and no one has even noticed my 280 ft horizontal loop.At some point i would like a 2m beam above it with its own feed and maybe a dipole under for low profile .So maybe not having easy access is a good thing as i would be chopping and changing and mending things that are not broken. Thanks for the input Kev and a HNY to all.

Ta Drew
M0TNX wrote:
Just a thought Drew..

What about an experiment, place a monoband dipole made of wire supported by fishing poles placed above your beam and on a seperate feed, give it a month or so and then go to the shared feed.. That way you can rotate the beam and dipole in a similar plane..

if you can?

Good luck and enjoy the experimenting.


Tue Jan 04, 2011 10:25 pm
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Heh-heh, the stealth approach to an antenna farm.

How is the mini beam going for you? Its something I've pondered getting from time to time.

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Wed Jan 05, 2011 12:25 am
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It took 2 years Kev for permission on the MQ-2 mini beam and thats because the photo i sent had one on a huge chimney that made the MQ-2 look small so must keep everyone sweet.
Conor it was the best compromise i could find it was between that and a cobweb.I can only compair it to my loop so would not be the best person to ask .20m is its biggest compromise and it is equal on there to the loop,17m the mini just beats it ,then 15-12-10m theres no compatition the mini is head and shoulders but the loop is crap on them bands . It works 6-2m but not great but not tested enough on them yet and the loop tunes and works them to ,2m is better on the loop .So overall its maybe equal to having dipoles and i dont have anywhere to fix dipoles.The cheaper way was with the Cobweb but my comunual antenna for the block is only 20ft and the MQ-2 has good front to side but next to no F/B. BTW Conor good luck with your gardening ,your neighbours must think your trying to grow your own wire.

Ta Drew


Wed Jan 05, 2011 9:00 am
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The next thing I'm going to try is asking the neighbour if I can stick a few radials under his lawn (well jungle). His jeep has been sat on my drive for 6 weeks cos the tax ran out and its waiting parts from USA so I think I'm in a reasonable bargaining position now. It'd improve things in a western direction immensely and I only need to go in 10ft :P

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Wed Jan 05, 2011 11:23 am
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Aye Conor only if you mow his lawn once a week and keep them weeds down. :) Good luck and no harm in asking .Drew
M0GVZ wrote:
The next thing I'm going to try is asking the neighbour if I can stick a few radials under his lawn (well jungle). His jeep has been sat on my drive for 6 weeks cos the tax ran out and its waiting parts from USA so I think I'm in a reasonable bargaining position now. It'd improve things in a western direction immensely and I only need to go in 10ft :P


Wed Jan 05, 2011 3:06 pm
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drewbhoy1 wrote:
Aye Conor only if you mow his lawn once a week and keep them weeds down.


Nah, thats what the kids are for, lol.

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Wed Jan 05, 2011 5:37 pm
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