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EI7GSB
06-26-2011, 10:38 PM
Hi,
I am about to put up a centre fed dipole 25 feet total length for use on 14 - 30 Mhz.
The feed line will be 300 ohm twin feeder and I have an Sg237 auto tuner which is unbalanced.
There will be about 10 feet of twinlead between the dipole and tuner.
I have 3 questions about this dipole configuration.
1. on 14 Mhz the total length is over 3/8 wavelength and I've read that efficiency should be greater than 90% of a full size dipole. Is this correct ?
2. If I tuned this dipole up on 10 Mhz, what relative efficiency could I expect ?
3. According to SG tuner documentation I don't need a balun at the tuner output, I just connect the ladderline or twin feeder to the output. This doesn't sound right and I think I need a 1:1 current balun between the twin feeder and tuner. If I do need balun, will something like a several turns of rg58 on a ft240-43 do the job or how should I homebrew a suitable balun.

Max power will be 100w ssb or 40w on digimode,
Thanks,
Ei7gsb

K8JD
06-26-2011, 10:43 PM
I would not expect a 25 ft dipole to perform well, much below 19 MHz !
z

W0VYE
06-26-2011, 11:02 PM
I just looked up that tuner's manual. They say quite specifically that the tuner should be installed at the feed point of the antenna.

KA4DPO
06-27-2011, 03:19 AM
It will work, I have done this with my AH4 several times and here's a webpage that explains it. http://home.comcast.net/~hamlakemn/AH-4_General_Information.html

You should not need a Balun but if you do get some Rf feedback issues (cranky tuning or RF in the shack) wind about ten turns of RG-8X, the flexible kind, on a piece of 4" PVC pipe and attach it inline with the coaxial cable to your radio. Be sure to place it as close to the tuner as possible.

G3TXQ
06-27-2011, 09:28 AM
The 3/8 wavelength "rule of thumb" is a useful one - your 25ft dipole will work fine down to 20m!

On 30m the dipole feedpoint impedance is getting problematic: the SWR on the twin feeder will be over 150:1 and the losses around 2dB over the 10ft run; the impedance at the end of the feeder will also be tough for the tuner to match efficiently, so expect more losses there. You'll probably get contacts if the tuner can match it, but be aware that you may be suffering several dB total loss.

A 1:1 current balun between the tuner and the feeder is the right engineering solution. I'd recommend 8 bifilar turns on a FT240-43 or FT240-31 core. If you were to try it on 30m the voltage between the bifilar pair will approach 3kV peak, so you will need to use wire with insulation that can cope.

73,
Steve G3TXQ

KA4DPO
06-27-2011, 05:38 PM
The 3/8 wavelength "rule of thumb" is a useful one - your 25ft dipole will work fine down to 20m!

On 30m the dipole feedpoint impedance is getting problematic: the SWR on the twin feeder will be over 150:1 and the losses around 2dB over the 10ft run; the impedance at the end of the feeder will also be tough for the tuner to match efficiently, so expect more losses there. You'll probably get contacts if the tuner can match it, but be aware that you may be suffering several dB total loss.

A 1:1 current balun between the tuner and the feeder is the right engineering solution. I'd recommend 8 bifilar turns on a FT240-43 or FT240-31 core. If you were to try it on 30m the voltage between the bifilar pair will approach 3kV peak, so you will need to use wire with insulation that can cope.

73,
Steve G3TXQ

Actually Steve a current Balun between the tuner and feeder is the wrong solution in this case. The tuner is floating so it will load into the twinlead just fine. Also, he never mentioned 10 MHZ (30 meters) he specifically said 14 through 30 MHZ.

Wthout question a center fed dipole is a balanced antenna. You can locate this tuner directly at the feed point without a balun, which by the way is not recommended by the manufacturer, and it will work just fine. When connecting parallel feeders to this tuner the worst thing that can happen is some feedline radiation. That will go somewhere so it's not like it's lost or anything. Any rate the place for a choke, either wound or ferrite sleeve type is at the radio side of the tuner.

G3TXQ
06-27-2011, 06:24 PM
Also, he never mentioned 10 MHZ (30 meters) he specifically said 14 through 30 MHZ.

I must be dreaming! So, what did he mean when he said: "2. If I tuned this dipole up on 10 Mhz, what relative efficiency could I expect ?"


Wthout question a center fed dipole is a balanced antenna. You can locate this tuner directly at the feed point without a balun, which by the way is not recommended by the manufacturer, and it will work just fine. When connecting parallel feeders to this tuner the worst thing that can happen is some feedline radiation. That will go somewhere so it's not like it's lost or anything. Any rate the place for a choke, either wound or ferrite sleeve type is at the radio side of the tuner.

I don't know the SG237, but I'm presuming it is not a balanced tuner; that means one of the output terminals - most likely the RF Ground terminal - connects directly to the input coax braid. If you don't use a balun, you have connected one side of the antenna (or feedline) directly back to shack ground via the coax. To prevent that you need a BalUn, and the right place for it is where the Balanced part of the system interfaces to the Unbalanced part i.e. the output of the tuner.

Without any balun not only can the parallel feedline radiate, but so can the coax; it can also pick up unwanted noise on Rx and inject it at the feedpoint.

There was a similar discussion on another recent thread; it's worth looking at Tom's comments:
http://forums.qrz.com/showthread.php?301909-Air-Wound-Balun-for-4-Conductor-Tuner-Control-Line

73,
Steve G3TXQ

M3KXZ
06-27-2011, 06:24 PM
DPO, you missed this line
"2. If I tuned this dipole up on 10 Mhz, what relative efficiency could I expect ?"

M3KXZ
06-27-2011, 06:24 PM
Steve....you beat me!

EI7GSB
06-27-2011, 08:35 PM
Steve....you beat me!
Thanks for all the replies and I guess we'll just suck it and see.
The tuner is actually the sg239 not 237 but its definitely an unbalanced type.
The manual says you can use it with centre fed dipoles at the feedpoint, centre fed dipoles with ladder line as well as the usual unbalanced longwires and verticals etc. I'll try it first with a coax choke wound on an ft240-43 on the tx side of the tuner. I'll then try a 1:1 current balun on the antenna side of the tuner. A small amount of radiation from the 10ft vertical feeder might actually make the pattern more onmidirectional but I don't want rf on the coax from the tuner back to the rig. I'll report back on my findings and thanks to everybody again.

G0GQK
06-27-2011, 09:39 PM
I assume the reason why your proposed antenna length is 25 ft is because that is the length available in your backyard. What you could do is make it longer and let the two ends hang down eg 25 + 10 +10 = 45 ft.

A number of years ago a G2 designed an antenna for limited space where the two horizontal legs either side of the feedpoint were about 10 ft, then there were vertical drops of wire, eg. down 5ft, 1 ft horizontal, 5 ft up, 1 ft horizontal, down 5 ft, 1 ft horizontal, then 5 ft up and so on and he made a dipole which would operate on 40 metres. The horizontal sections near the ground were separated with thin garden canes. This design was around long before the internet and was featured in a magazine so a search won't find anything like it, but it worked.

EI7GSB
06-27-2011, 10:05 PM
I assume the reason why your proposed antenna length is 25 ft is because that is the length available in your backyard. What you could do is make it longer and let the two ends hang down eg 25 + 10 +10 = 45 ft.

A number of years ago a G2 designed an antenna for limited space where the two horizontal legs either side of the feedpoint were about 10 ft, then there were vertical drops of wire, eg. down 5ft, 1 ft horizontal, 5 ft up, 1 ft horizontal, down 5 ft, 1 ft horizontal, then 5 ft up and so on and he made a dipole which would operate on 40 metres. The horizontal sections near the ground were separated with thin garden canes. This design was around long before the internet and was featured in a magazine so a search won't find anything like it, but it worked.

Yes there is a reason for the 25 ft. I have a 20m long off centre fed dipole for use on 40-10m but its running east west, firing north south on 20m which I don't want, I want east-west, on 15m it seems to have large nulls to the south and possibly other nulls directly east west. The windom is also a bit noisy. In my attic I have fan dipoles running north-south for 17, 15 and 10m. ( firing east-west) These dipoles, even with 1 -2 S point signal loss in the attic have much better signal to noise ratio on Rx and I can hear stuff on these attic dipole that are lost in noise on the outside windom. So my thinking was to replace the attic dipoles with a single dipole with remote autotuner over the roof of the house in the same orientation and get the better tx and Rx than the windom on 20-10. ( I'm not too interested in 40m because of the arrival of a new plasma tv or other offending electronic junk in the neighbourhood which makes operation there tedious) I have only 25ft available over the house for this dipole but possibly could add a few feet on either side if I drop the legs down the support poles, which will be fibreglass when they are finally installed. So I'll certainly try out your suggestion, thanks EI7GSB

G3TXQ
06-28-2011, 08:29 AM
Dropping 10ft tails at the ends is unnecessary - the impedances without it are perfectly acceptable. But worse, adding the tails reduces the effective height of the antenna by placing a significant portion of the current lower down, and it rotates the azimuth pattern; it's particularly noticeable on 10m where, at low take-off angles, the broadside gain would be reduced by 8dB; that's something you don't want in an antenna you have carefully orientated to "fill in" particular directions.

Your "3/8 wavelength" rule of thumb is sound advice; it's a myth that you have to make the dipole a half-wavelength at the lowest operating frequency, and if you try to achieve it in the way G0GQK recommended it can have undesirable consequences for multiband use.

Here are the comparative azimuth patterns at 10 degrees take-off angle; Red is 25ft dipole; Blue is effect of adding 10ft tails:

http://www.karinya.net/g3txq/temp/dipole_tails.png

73,
Steve G3TXQ

EI7GSB
06-28-2011, 09:03 AM
Dropping 10ft tails at the ends is unnecessary - the impedances without it are perfectly acceptable. But worse, adding the tails reduces the effective height of the antenna by placing a significant portion of the current lower down, and it rotates the azimuth pattern; it's particularly noticeable on 10m where, at low take-off angles, the broadside gain would be reduced by 8dB; that's something you don't want in an antenna you have carefully orientated to "fill in" particular directions.

Your "3/8 wavelength" rule of thumb is sound advice; it's a myth that you have to make the dipole a half-wavelength at the lowest operating frequency, and if you try to achieve it in the way G0GQK recommended it can have undesirable consequences for multiband use.

Here are the comparative azimuth patterns at 10 degrees take-off angle; Red is 25ft dipole; Blue is effect of adding 10ft tails:

http://www.karinya.net/g3txq/temp/dipole_tails.png

73,
Steve G3TXQ

Thats great stuff Steve, Ok I'll stick with the 25 ft for 14-28Mhz and at least I'll be firing the max gain in the right direction.
I've other antenna that I can use on the lower bands.
Thanks Again, Ei7gsb Ciaran

W8JI
06-28-2011, 02:22 PM
Any rate the place for a choke, either wound or ferrite sleeve type is at the radio side of the tuner.

That is absolutely wrong. It makes no technical sense to do that with any unbalanced tuner, and it really complcates things with atuner having control lines.

It is a bad idea based on incorrect analysis of the system and what a balun does.


Regardless of what SGC or anyone else says to do, the balun belongs on the OUTPUT side of the tuner.

http://www.w8ji.com/tuner_baluns.htm

73 Tom

KA4DPO
06-28-2011, 05:18 PM
That is absolutely wrong. It makes no technical sense to do that with any unbalanced tuner, and it really complcates things with atuner having control lines.

It is a bad idea based on incorrect analysis of the system and what a balun does.


Regardless of what SGC or anyone else says to do, the balun belongs on the OUTPUT side of the tuner.

http://www.w8ji.com/tuner_baluns.htm

73 Tom


Since when is it a bad idea to put a choke on the coaxial cable comming from the antenna system to keep common mode current from flowing on the shield and having RF feedback problems in the shack?

As for putting a balun on the antenna side of the tuner, it is completly unnecessary and not recommended by the manufacturer. There is a much higer probability of having RF on the coax to the rig and that is where a choke is desired. There is absolutely no penalty by connecting the autocoupler to a short balanced feed line, none and putting a balun at the feed point is a waste of money and will have almost no (understatement) effect on performance.

K8JD
06-28-2011, 05:19 PM
I don't recall hearing this 3/8 wave rule until now. I just think it's pushing things a bit.
I just put up a 90 ft dipole (using balanced line feed for the first time) . It does not work on 80 M worth a hoot in hades ! It works very well on 40/30/20 M, compared to some of my other vertical and wire asntennas. .
mY 80m half wave dipole works 100 percent better than this "3/8 wave" antenna even with 250 ft of RG8X line.
Only my own observation, your milage may vary !

W8JI
06-28-2011, 05:29 PM
Since when is it a bad idea to put a choke on the coaxial cable comming from the antenna system to keep common mode current from flowing on the shield and having RF feedback problems in the shack?

As for putting a balun on the antenna side of the tuner, it is completly unnecessary and not recommended by the manufacturer. There is a much higer probability of having RF on the coax to the rig and that is where a choke is desired. There is absolutely no penalty by connecting the autocoupler to a short balanced feed line, none and putting a balun at the feed point is a waste of money and will have almost no (understatement) effect on performance.

If SGC tells people not to use a balun on the antenna side of the tuner when feeding a balanced antenna, they are absolutely wrong. That is terrible advice.

If you put a common mode choke on the shack side of the tuner, common mode currents simply move to any control or power cable and the mast.

The WRONG place for balun or common mode choke, when a tuner feeds an unbalanced load, is on the radio side of the unbalanced device.

73 Tom

KL7AJ
06-28-2011, 05:29 PM
One very effective 10 meter antenna that I've had up for ages is s folded dipole fed with 300 ohm twin lead with a balanced tuner (4:1 balun output). Although there's a small mismatch, it's quite low loss....much less than feeding a "normal" dipole with 300 ohm twin lead.

Eric

EI7GSB
06-28-2011, 09:04 PM
I don't recall hearing this 3/8 wave rule until now. I just think it's pushing things a bit.
I just put up a 90 ft dipole (using balanced line feed for the first time) . It does not work on 80 M worth a hoot in hades ! It works very well on 40/30/20 M, compared to some of my other vertical and wire asntennas. .
mY 80m half wave dipole works 100 percent better than this "3/8 wave" antenna even with 250 ft of RG8X line.
Only my own observation, your milage may vary !

THe 3/8 wave minimum length on lowest band is found in many antenna references, Cebik has been quoted as one of the references I believe. Anyway its a multiband antenna and is 3/8 wave on 20m only, 1/2 wave on 17m which is one of my favorite bands and greater than 1/2 wave on 15 and 10m which are starting to get more active now. To be honest if there is 2-3dB loss total at 20m over a 1/2 wave dipole (which I don't think there will be ) I don't really care as I want this antenna to fill in the nulls that are apparent on my windom antenna which is pointing in the wrong direction for me and the windom is noisy too.
The antenna is up tonight but I still have to wire up the tuner and run the coax etc.
When I get the chance I'd like to do some comparisons with the windom antenna with stations from different directions. As I'm an engineer I'd like to do a semi-scientific study of placing the balun at each side of the tuner and at the same time do A / B comparisons of the new antenna against the windom but I'm unlikely to have time to do all that. But thanks again for everybody's input and its great to see the various agruments that are being put forward. 73 EI7GSB Ciaran

KB4QAA
06-29-2011, 05:56 AM
I had never heard the 3/8L rule of thumb until last month in this forum. I don't dispute it. Just never heard it before.

Bill

EI7GSB
07-01-2011, 02:56 PM
I had never heard the 3/8L rule of thumb until last month in this forum. I don't dispute it. Just never heard it before.

Bill


In the Radio Communications Handbook published by RSGB, chapter 15 - by Peter Dodd. He states that
for a open wire tuned dipole - " a 3/8 wavelength dipole has an effectiveness greater than 98% relative to a half wave dipole".
He also says that this size is quite easy to tune and states that when you get down to 1/4 wavelength top length
its gets beyond the tuning range of almost all tuners.

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