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26GB82 Profile
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Registered: 02-2009
Posts: 31
Karma: 1 (+1/-0)
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posticon Using a beam on additional bands...


I'm going bonkers here listening to stations on 17 metres and not on 11 to the point that today I've embarked on another Wickes Moxon this time cut to 18.150.

While I've only built the boom at the moment and that's looking a bit hefty I'm hoping that by magic the new beam will work on 11 metres as well as 17.

The elements are going to be 19 1/2 feet wide this time and the boom is already 7 1/2 feet long,again it's a two element(driven element plus reflector).....I think I can hear the neighbours putting on their war paint but my reason for this post is does the team think that the larger dimensions plus an ATU will let the antenna perform similarly to a conventional two element 11 metre Yagi?

Without building a new antenna but using an ATU to match the existing 11 metre beam I can't help thinking the reduced efficiency of an undersize antenna used on 17 metres will be ultimately more frustrating than the route I'm now taking although it's less than perfect.

Suggestions,thanking you please,Dave.
14/Feb/2009, 7:03 pm Link to this post Send Email to 26GB82   Send PM to 26GB82
 
Hot Fusion Profile
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Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
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Re: Using a beam on additional bands...


I would guess that it won't work at all well, but that is only a guess based on theory, and many of us know that sometimes antennas don't always work strictly to theory, right?

The biggest problem I see is the need to use an ATU. Remember that an ATU only presents the rig with the correct load impedance, it doesn't make the antenna correct and doesn't tune it at all. (That's assuming you are using a co-ax fed antenna, not a dublet/long wire.)

What will happen is that you will have a huge (but unseen) SWR on the co-ax which will make for a lot of efficiency loss - very little of the energy will actually make it into the driven element.

That's my view of it, but it will be interesting to hear other views as always.

---
Andy - 26FB107
15/Feb/2009, 5:03 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
26GB82 Profile
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Registered: 02-2009
Posts: 31
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Re: Using a beam on additional bands...


Cheers Andy,here's another thought I had while bashing around at work today, what about traps to allow for dual band operation.....I'm making the beam to 17 m dimensions which automatically means the elements are plenty long enough to work on 11 m too although the rather long spacing to the reflector when working 11 m at 7 1/2 ft away is going to produce a non 50 ohm match.

If you could kindly dash off some lumped values for me as I'm a hammer and nails merchant who only gets to grips with the maths of the problem as a last resort.

I did also think about using relays to alter the electrical lengths of the elements but I've worked three rather long days in a row at this point so if nailing hamsters to them was suggested that might also be considered.

Either way construction will recommence tomorrow although I am getting misgivings as this is looking to be a bit of a big burger.
17/Feb/2009, 11:58 pm Link to this post Send Email to 26GB82   Send PM to 26GB82
 
John193 Profile
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Freebander

Registered: 11-2008
Location: South Glos
Posts: 260
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Re: Using a beam on additional bands...


The efficiency will be diabolical at best and as usless as tits on a bull at worst. A beam by nature is not braodband and if you take a look at comercial stuff you'll see they actually stick separate antennas on the same boom or trap the radiators and parasites. Even a quad which will be res at all harmonics will not give you a beam but 2 lobes at second harmonic side lobe at third and it gets worse. The feeder will not accept even accept 10% of radiated energy so guess what it comes back as high swr but unreadable because your scale wont go high enough!!!! try separate antenna on same boom that will work but you'll be all day getting it tuned due to that devil mutual impedance!. Good luck. Johnfb193 emoticon
24/Mar/2009, 10:19 pm Link to this post Send Email to John193   Send PM to John193 MSN
 
26fb99 Profile
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Accomplished DX-er

Registered: 08-2008
Location: Stoke on Trent North Staffs
Posts: 341
Karma: 4 (+4/-0)
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Re: Using a beam on additional bands...


jhon youve been real busy answering all the questioned ive been holding on to but for other lads.
just opened up my yaesu817 for wideband TX so as a 100% mobile/portable station(qrn from broadband line adaptors at my home qth),and sitting my foundation in a few weeks.any ideas on an aerial system using my current stuff(see below) and a good amateur atu or just get a g5rv? would prefere the benefits of my quad on 80 and 20m or is this not happening?cheers for any advice.

---
yaesu FT857d,rmkl500,wilson5,2element pubical quad(the BBq).split charger aux battery.170a truck battery,mfj 941 versatune2
100% mobile station
I got 99 Problems but a whip aint 1
Brent 26 Fox Bravo 99
73s
24/Mar/2009, 11:38 pm Link to this post Send Email to 26fb99   Send PM to 26fb99
 
John193 Profile
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Freebander

Registered: 11-2008
Location: South Glos
Posts: 260
Karma: 3 (+3/-0)
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Re: Using a beam on additional bands...


I'm sorry to be the bearer of bad news but the pattern from a quad on just the second hamonic looks like a clover leaf but with bigger gaps! I have searched my whole life for the doitall antenna it just basically doesnt exist. Everything is a compromise bandwidth v's gain v's phys size v's feedability v's cost, turning circle, windsurvival, weight the list is endless as are the as are the variations. To decide on the best antenna for A job thats A job prioritise the requirement half the gain, double the work involved halve the dream double the cost and the ideal antenna is staring at you! probably a dipole for every band! or sky loop is good alrounder. check out my experimental double5/8 colinear phase shifted contraption, its huge unweildy very tight banded useless EXCEPT ON 27555 WHERE ITS AN OMNI BEAST, ie its good for one thing only! nuf said? TTFN emoticon
25/Mar/2009, 9:09 pm Link to this post Send Email to John193   Send PM to John193 MSN
 


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