You’re right, of course… here is a comparative measurement from DL2LTO… for 30m feed line in this way.
With 20m it is just about possible…if the line is not too long
73 Armin
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I always build my antennas down in the yard or park… and when they work I go with them to the summit 
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…by the way do you know the great video from Ingacio @EA2DB
Armin
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As promised earlier I follow-up with some fotos.
Here are some dipole centerpieces made from strips of plastic.These are all recycled lids of food containers. Philadelphia cheese and Pringles chips come with suitable plastic lids.
This is the most fancy one built recently. It has a real strain relief to the dipole legs. I have used pieces of silicone tubing pushed over the feeder to prevent it from breaking. Not shown here is a final layer of heat shrink tubing on the crimped connectors.
In my experience these dipole centerpieces last at least for 100 activations. If they get older the wire might break. It is a good idea to use a cold winter evening to refurbish your dipole antennas once a year and replace centerpieces, shorten the wires by a few inches and make all connections new.
This is my connection of the feed line to a KX2.
Plenty has been discussed if a balun is necessary or if it offers any advantage. I have decided to go without a balun. After all, where is the unsymmetry? One side of the dipole is connect to the enclosure of your rig via the ground connection, the other isn’t. Does not look like a big assymetry to me. It would be different if the rig was operated from a mains power supply. But this is anyway not the case for a SOTA activation.
73 Heinz
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Scabbing covers the full range of procuring without payment from begging to dumpster diving, but stops short of doing something likely to result in a conviction, if anybody is watching.
In some quarters there is a spiritual aspect as scabbed objects have mauri that is lacking from bought and new objects
also noun: The pull tag or crown seal on a beer . “Pick the scab off another Lion Red”
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In some cases two maybe ok.
For your case the 4:1 Transformer (first of the three) is not relevant.
73, Ludwig
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There is one more question. Do you use an ATU (matching unit, extra device or part of your TRX)?
case 1 - no, case 2 - ATU only usable for lower antenna SWR (up to 3 or 4 or so) or case 3 - for higher antenna SWR (up to 10 or more).
case 1: you have to tune your antenna (mechanical) together with the feeder by using a SWR meter or better an antenna analyser.
case 2: a well calculated antenna may work well without tuning by the cutter.
case 3: nearly all random wires are usable.
73, Ludwig
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Hi Armin,
At the risk of confusing Joe and being labelled a PITA, your 2:1 balun isn’t accurately named It is a 1.5:1 current transformer and matches a 112 5 ohm load to 50.0 ohms source. It is more elegant than the 2:3 turns ratio voltage transformers I have used in the past.
Thanks for posting it.
73
Ron,
VK3AFW
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That’s the fantastic thing about the KX2’s tuner… it matches almost anything: whether it’s open lightning rods, cut aluminum foil, or just a long wire with a counterpoise…
In general, without a tuner, (when you have tuned the system) you will have an SWR of 2 - 3 without balun.
But: Normally this does not kill the power amplifier.
But as I explained above, I’m not a fan of using dipoles on summits anyway…:
- In my experience dipoles are much more sensitive than endfeds as far as ambient influences are concerned. (especially if you don’t have a tuner, you are quickly in the border area of SWR). And there are different conditions for each summit.
- A Balun 2:1 transforms here also the influences and lets them become a little smaller.
- I can set up the endfed more diversely (Slooper, L, Inv V. …).
With a balun 49:1 to 64:1 the influences become even smaller. - And since I’m lazy, perhaps the most important argument for me: for a band change on a linked dipole, I have to get up and lend a hand … maybe even lower the mast.
But that is my philosophy… 
73 Armin
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Hi Armin,
I don’t understand why an end fed half wave dipole would be less affected by ground variation than a centre fed half wave.
Either antenna in an inverted vee configuration has the current maximum as high as possible and therefore should radiate best with no measurable difference.
The advantage of the 40 m half wave end fed is that it can be readily used on 40, 20 and 15 m from the operating position which is good as they are the most productive bands overall.
It can also have a shorter run of coax which might be convenient in terms of loss at the higher bands and takes less space.
The downside is that the popular designs use the coax, the rig and the operator as part of the antenna. Not hard to fix but hard to convince people who use them.
73
Ron
VK3AFW
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Hi Ron
well… these are my experiences now:
When I set up a dipole with feeder line in different terrains, sometimes stretched, sometimes as inv V, sometimes as slooper (whatever space allows), I have always had strongly different SWR .
I don’t think that only the distance above ground is a criterion… it could also have been trees or towers or …
With the Endfed the influences were not so strong. And of course I have about 0.05 lambda coax cable from the 1:49 balun to the W1JR balun. But again, I can leave the coax pretty much uncritical… in bows… on the ground…
My idea is that the changes in the antenna don’t affect it as much because it is attenuated via the 1:49 transformation.
Most of the time I don’t mind all that, because the tuner from the KX2 as compensates. However, I am also active with the QCXmini or a small PA without tuner … and there I need an adapted antenna.
73 Armin
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Hi Armin,
Maybe its the same thing like described here (second balun). When using symmetrical feeding lines there are similar effects with common mode currents. When the feeder acts as a radiator (coomon mode current) the situation around the feeder affects the SWR.
73, Ludwig
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Well, I was already confused about this. How can I know how many ohms are in my antenna or feed line? I know ohms are a measure of resistance, and also impedance (? here’s where I’m a bit fuzzy on the concept) and I know how to measure ohms through a resistor but I don’t know how to measure the ohms in/at a feed line. So, how can I know whether I need 2:1 or 1.5:1 or 4:1 or whatever?
As expected everyone has given their $0.02 and confused the pants off you.
There are 2 kinds of half wave dipoles… centre fed and end fed. Centre fed is easiest as it 2 lengths of wire and some coax. Its impedance will be sort-of right for your radio. You can make some kind of balun and see if it makes a noticeable change. For QRP HF SOTA, RG-174 is more than good enough coax and just about any old thin wire will do.
An end-fed is exactly the same apart from it’s fed at the end and you need some gubbins (inductor/capacitor) to match the antenna’s impedance. It works the same. But if you’re a noob then it’s harder to make as you have to make (not buy, make) the match unit. It’s not hard but play with a centre fed first. Learn how it works. Then make your endfed. See how it works the same but can be rigged into the air differently. Play with it.
When you have made a few you can get adventurous and try verticals and multi band and … and … and… all sorts of magical stuff.
Make a coax centre fed first as it is the most likely to work first time. Do you need a balun, theory says yes. Practice says maybe. So make a balun out of 5-10turns of the coax feeder. You can see if it makes a difference or not.
The make an end-fed and play with different match units. Walk then run. Then you can try balanced feed instead of coax. There is lifetime’s worth of fun to be had playing with simple bits of wire.
Just remember no antenna is perfect. But any antenna is better than no antenna.
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Confusion sowed. Our work here is done.
They have wandered off topic. It’s congenital. Your sympathy and forbearance are appreciated.
A straight dipole, way up in the air is 70 ohms. It just is.
An inverted V dipole, which is how you are going to pitch it on one pole, is closer to 50 ohms, so you are not wanting an impedance transformation.
You want a simple (1:1) current choke balun i.e. you wrap the transmission line through a ferrite core 5 times or more. There are some nice pictures above.
It doesn’t need a box - ferrite doesn’t rust.
Just to be clear. 5 turns of coax through a ferrite ring/sleeve is a balun.
5 turns of coax in air, is not** a balun at 40m (but it is at VHF eg 146MHz)
** Ok, so I just asserted that, and thought I would check. 5 turns on 100mm diameter = 4.5uH → XL@ 7MHz=198 ohms. This would reduce the current down the outside of the coax to 1/4. That would be enough to reduce the effect on antenna pattern significantly. The radio would still be “hot” - which only really matters at high power. Any switchmode/computer noise at the radio would be almost unaffected by this.
So it would likely be useful in the field (better than not doing it), and a very poor choice for home station.
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Great discussion. I do have an EFHW that i made at home for my 100W base station, but by “dipole” in this thread I’m specifically asking about the center-fed type. (I guess all antennas are dipoles, technically.) I do have a bit of experience with soldering and kits, and have wound a few toroids, but I don’t have the electrical knowledge to design my own.
I made a linked dipole for the 40,20,15 and 10 meter bands, but you could do whatever bands you wanted. I used rg-174 for the coax, wound that around a 114-43 ferrite. Attracted the ferrite to a piece of plastic, with a hole in the center for a mast. I used thin speaker wire for the dipole legs, and Anderson powerpole connectors for the dipole links.
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Hi Joseph,
you may want to have a look at How to Build and Tune a Linked Dipole Antenna.
Charly, NJ7V has explained everything in detail. The construction is pretty easy and you can add a choke if you would like to and also design the links for other bands. The calculator on the SOTA maps page gives more than sufficient extra lenght for the connectors.
My version of the linked dipole is definitely not optimized regarding weight but fits easily into a small box.
73, Peter
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Wow - so many posts, so much advice. 
For me, I have been using linked dipole antennas for more than 10 years.
http://vk3zpf.com/link-dipole
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