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 88ft doublet 
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Post 88ft doublet
I intend errecting a doublet for 80M, probably around the 90ft mark will be the maximum I can cram into my site.
I have been reading up on the use of ladderline and baluns as well as feeding such antennas with coax or even 75ohm twin feeder
Am I right in thinking that the impedence at the feedpoint |(of the above intended antenna) will be manageable via an unbalanced ATU or even onboard ATU without a BALUN and is it acceptable or recommended to feed this antenna with coax. Note I only require single band 80M coverage and the coax length will be less than 7M, probaly using mill spec RG8 mini.

Am I right in thinking that if I could feed the antenna with 44 ft of balanced feeder this woudl improve the efficency of the antenna??
I cannot get this amount of feeder in free space, hence my leaning towards feeding with coax fore the one band i.e 80M

Thaks in advance for help/advice etc....
Ian M0IOD


Mon Jan 18, 2010 11:03 pm
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 88ft doublet 
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Post Re: 88ft doublet
88ft is pretty short for a doublet on 80m - at a height of 30ft its feedpoint impedance is around 17-j670 ohms; that's a VSWR of 500:1 in a 50 ohm system.

You could expect losses of about 5dB - 70% of your power - feeding it directly with 7m of RG213; I don't have the loss figures for RG8 mini, but I presume they are worse than RG213. The impedance at the tuner end of the coax will be way outside the matching capability of a radio's internal tuner - even a good external tuner may struggle!

If you fed it with 7m of 450 ohm ladderline, the feedline loss would drop to 0.8dB - 16% of your power - and the impedance would be suitable for an external tuner, but not an internal tuner. You would need to use a 1:1 current balun between the tuner and the ladderline. You should not use a 4:1 balun - the resistive part of the impedance is already very low, and the last thing you want to do is divide it by 4!

I don't think there's much advantage in using 44ft of ladderline if the 7m will reach. The loss increases to 2dB, although the impedance becomes slightly "easier" for the tuner to match; but it's still outside the range of a radio's internal tuner.

No transmission line, of whatever length, can transform the feedpoint impedance of that doublet to a value which is within range of an internal tuner.

Hope that helps.

Steve G3TXQ

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Tue Jan 19, 2010 9:17 am
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 88ft doublet 
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Post Re: 88ft doublet
hi ian , here's my opinion for what it's worth if i had to get an 80m monobander up in the same situation.


load the antenna to an electrical halfwave /coax feed

steve will probably be able to give much more help but i think id go with the loading myself

billy

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Tue Jan 19, 2010 9:32 am
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 88ft doublet 
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Post Re: 88ft doublet
Billy,

Thanks for the reminder! Yes - inductive loading is an option. EZNEC predicts that loading an 88ft doublet in the middle of each leg will give you a low 1.5:1 VSWR, so coax feed is then an option and you can use the radio's internal tuner.

The only issue is that it becomes relatively narrow-band; the VSWR will be over 10:1 at the band edges. However there should be a 150kHz "slice" mid-band where the VSWR is below 4:1 - low enough that it can be used with the internal tuner.

I guess it depends if that is satisfactory for you - if so it's the best option. If not, ladderline and an external tuner give you more flexibility, including the ability to work all bands 80m thru 10m should you want to in the future.

73,
Steve G3TXQ


Tue Jan 19, 2010 10:54 am
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 88ft doublet 
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Thanks it looks like an 88ft(or as long as I can extend each leg) loaded dipole will be the way forward. Just need to calculate how to form the coils for each end, gauge of wire and diametre of coil as well as how many turns. Looking foor resonance around the 3.750mhz mark or thereabouts.
I will be feeding with coax as I only need 80M coverage.
Does anyone know of any software orf sites to calculate the coils which will be loaded at the end of each leg.

Cheers Ian


Tue Jan 19, 2010 4:43 pm
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 88ft doublet 
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Post Re: 88ft doublet
try here ian..
http://www.k7mem.150m.com/Electronic_No ... rtant.html
billy

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Tue Jan 19, 2010 4:46 pm
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 88ft doublet 
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Thanks Billy appreciated


Tue Jan 19, 2010 4:51 pm
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 88ft doublet 
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Post Re: 88ft doublet
Ian,

The K7MEM site predicts you need about 20.7uH coils and EZNEC predicts 23uH - probably neither will dead right so be prepared to do some pruning. I'd go for slightly more inductance and then trim back the outer wire sections in equal amounts.

You're building quite a 'high Q' structure so small changes in inductance will shift the frequency a lot, as will small adjustments to the outer wires: a 0.5uH change in the coils will shift you about 30kHz, and 6" off each of the outer wires will shift you about 50kHz.

Good luck, and let us know how it goes.

Steve G3TXQ

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Tue Jan 19, 2010 6:11 pm
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 88ft doublet 
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Post Re: 88ft doublet
Ian could you hang 20 feet or so off each end giving you a half wave "inverted U" shape?
Not sure what impedence you would see at the feed point, I'll leave that to Steve to tell us :wink:

John - G7SSE


Tue Jan 19, 2010 7:48 pm
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 88ft doublet 
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Post Re: 88ft doublet
JohnG7SSE wrote:
Ian could you hang 20 feet or so off each end giving you a half wave "inverted U" shape?
Not sure what impedence you would see at the feed point, I'll leave that to Steve to tell us :wink:

John - G7SSE


An almost perfect match mid-band - 49 ohms - and complete coverage of 80m with an internal autotuner :)

Steve G3TXQ

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Tue Jan 19, 2010 9:41 pm
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 88ft doublet 
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Post Re: 88ft doublet
coax length stated as around 7 m i dont think it would be high enough.

billy

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Tue Jan 19, 2010 10:24 pm
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 88ft doublet 
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Post Re: 88ft doublet
m0jha wrote:
coax length stated as around 7 m i dont think it would be high enough.

billy

Unless the antenna is 30ft up and the 7m is a horizontal run into a 4th floor shack window ;)

Seriously, one of the problems of trying to answer these sorts of questions is our not knowing the specific environment nor what might be acceptable to the OP. I guess all we can do is set out the options with their pros and cons and let him decide.

73,
Steve G3TXQ

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Tue Jan 19, 2010 10:40 pm
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 88ft doublet 
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Post Re: 88ft doublet
G3TXQ wrote:
m0jha wrote:
coax length stated as around 7 m i dont think it would be high enough.

billy

Unless the antenna is 30ft up and the 7m is a horizontal run into a 4th floor shack window ;)



:lol:

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Tue Jan 19, 2010 10:49 pm
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 88ft doublet 
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Post Re: 88ft doublet
m0jha wrote:
coax length stated as around 7 m i dont think it would be high enough.

billy


If not another option would be to dogleg the extra at the ends horizontaly, if Ians location could accomodate that.

I was also wondering about the feeder length Steve, thinking unless Ians shack is directly under the feedpoint with the feeder going stright up he isn't going to have a lot of height, though as you say his shack could be on the fouth floor :D

John - G7SSE

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Tue Jan 19, 2010 10:54 pm
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 88ft doublet 
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Post Re: 88ft doublet
Now then I have done a site evaluation of the space available to me and have several options, just wondering what will be the best way to go.
Remember I am looking to achieve maximum performance on 80M only. I have other antennas for other bands I use so am not unduly concerned about multi band performance.

I can get a 88ft loaded dipoel, one leg out in a straight line, this will be approx 25ft off the ground,fed with coax via a 1:1 balun.The other leg will run 23 ft then deflect at a 45 degree angle for around 19ft.

Other alternatives which I am considering are a 130FT WINDOM.
The feedpoint would be apoprox 20ft off the ground. The short leg would run in a straight line for 23ft or so to an elevation approx 35ft above the ground it would then then bend at 45 degree angle or so and run to the fixing point which will be approx 25 ft off the ground. My reasoning is that I get a fair straight run out. Construction is e alittle more complicated and I am a little dubious as a friend of mine installed one and found it to be fairly noisey on RX.
The longer leg would run approx 50 ft in a straight line from the feedpoint gaining height to an insulated tie off point approx 25 ft off the ground. The leg would then turn at 90 degrees and run for approx 40 ft to the fixing point again 25 ft off the ground.The windom would be coax fed via a 4:1 balun. The run of coax would be no more than 7M

Second option is a dipole(My gut feeling is that I would be best advised trying this antenna). This would be fed at the feedpoint again 20 ft or so off the ground.(I intend coax cable to a 1:1 balun) unless advised otherwise. Leg A would agin follow the 23ft as per the windom to a tie off point 35 ft up. The leg would then once again deviate at 45 degrees for another 23ft before another deflection of around 30 degrees running for 20 ft to the tie off point which would be approx 20 ft off the ground.The run of coax would be around 7M. The only thing that concerns mme is the deflections that the legs take and the fact that it is not out straight.How detirmenintal will this be?

I could also treat such an antenna as a doublet and feed with balanced feeder, it would of course be in the same weird configaration.

Last option is to use a 9:1 unun and a longwire fed against ground or a counterpoise. Question is which do you think will yield the best results. I am minded to go down the dipole route, even though it is far from in a straight line and will no doubt have its own limitations. I currently use such an antenna on 80M(longwire with UNUN) and it performs but not that well. Do you think that I should go down the dipole route and even though bent is it anticipated that such a dipole in such a configaration would be a better bet than my longwire or a loaded dipole in a straight line.

Thanks for all of your help and advice

Ian M0IOD

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Thu Jan 21, 2010 10:24 am
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