"Portable" Antennas

Started by KC9TNH, June 18, 2013, 03:08:46 pm

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KC9TNH

Welcome Davide, very interesting, all the information presented. A SOTA person I've had contacts with before swears by his telescoping jackite pole, and a dipole fed with 300-ohm twin lead. He very seldom sounds "QRP" even though he is.
72

KK0G

I think the reason a lot of hams are reluctant to use  balanced feed line is because they're scared from all the horror stories they've heard. Most bad things that a majority of hams have heard regarding balanced line are either blown way out of proportion or just dead wrong. Balanced feed line is lightweight, very low loss even with an obscenely high SWR and best of all it's dirt cheap compared to most coaxial feed lines. I've been feeding my main antenna here at the shack - a 140' dipole - with cheap 300 ohm TV twin lead for about 10 years now........... you couldn't get me to replace it with coax if you gave it to me for free.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

KC9TNH

Quote from: KK0G on September 08, 2013, 06:42:50 pmI've been feeding my main antenna here at the shack - a 140' dipole - with cheap 300 ohm TV twin lead for about 10 years now........... you couldn't get me to replace it with coax if you gave it to me for free.
If that's what you're normally using with your 3w to me I can attest that is one barn-stompin' antenna.
;)

KK0G


Quote from: KC9TNH on September 08, 2013, 06:56:38 pm
Quote from: KK0G on September 08, 2013, 06:42:50 pmI've been feeding my main antenna here at the shack - a 140' dipole - with cheap 300 ohm TV twin lead for about 10 years now........... you couldn't get me to replace it with coax if you gave it to me for free.
If that's what you're normally using with your 3w to me I can attest that is one barn-stompin' antenna.
;)


That's the one.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

gil

QuoteTheir weak point is that the RF energy is carried outside the line and not confined within the shield: this means that they interact with every object they encounter and their path must be carefully planned.


My issue would be getting the line inside the house. Right now I use RG-174 and it goes through an aluminum window frame where it makes two 90deg bends. Very bad, I know... Due to the aluminum, could I route open line the same way, or would I be better off going through the wall? I do have two 50-Ohm 8" barrel plugs that I could use to turn the balanced line into coax, but what would I need to do and would that even be  good idea? The open line would have to turn into a short coax somewhere, right? What if the tuner is inside the radio?

Gil.

KC9TNH

For me there is that RF issue at a fixed location; it's a personal nit that I pick. With the various wires that end up in my shack I definitely prefer coax.  But they do make great portable outfits where proximity to metallic and other such objects aren't an issue. What has been told to me about the twin-lead is: don't loop it, don't let it lie ON the ground (draping across the tops of bushes/tumbleweeds totally allowed).  The folks I know that use it regularly come to a very short length of coax, a jumper really, via a 1:1 or 4:1 current balun (ratio depending on length or config of the antenna). The SOTA guy I mentioned before runs 44' per leg, sometimes Inverted-V but flat-topped as much as he can reasonably do it, then about 30' of twin-lead to a baby-sized 4:1 current balun.  That's just one way to do it I guess. He is running a KX3 with ATU by the way, and runs 80-10 with that.

Perhaps Davide can speak some more to that or KK0G on how that gets handled for the fixed location.

IZ2UUF

Quote from: KC9TNH on September 08, 2013, 06:34:42 pm
Welcome Davide, very interesting, all the information presented. A SOTA person I've had contacts with before swears by his telescoping jackite pole, and a dipole fed with 300-ohm twin lead. He very seldom sounds "QRP" even though he is.


I really believe that: the antennae commonly used for QRP activity are so inefficient that something as simple as a dipole gives unbelievable results. The classic 30 ft "fishing pole" antenna in 40m gain is -10 dBi or less, never mind those 5 ft "miracle" sticks.

73 de Davide IZ2UUF

gil

QuoteI really believe that: the antennae commonly used for QRP activity are so inefficient that something as simple as a dipole gives unbelievable results.


Depends on what you call a "commonly used QRP antenna..." If you mean a Buddistick or something similar, sure, though they do work. I use end-feds, and they are as god as a dipole. In fact they are dipoles of a sort, end-fed dipoles. Performance is really a matter of size (that's what she says ;-), no escaping that. The only exception might be the magnetic loop, but you get extremely narrow bandwidth as a price.

Gil.

Archangel320420

From my home station when I was a novice and general class way back when, I used an end-fed zepp that was 65 feet in length and fed it with 300 ohm open wire ladder line, the old style ladder with glass insulators and heavy wire. I ran a KW from it once. It worked very well on 40 meters  and up. I have used the same length wire using 300 ohm TV twin lead using lower power and it works the same. Fantastic. 300 ohm tv line can be gotten at Radio Shack :) Just don't bother telling them why you want it. You will be in for a whole lot of nonsense talk about antenna installations and why you should buy coax line instead, unless your guy at Radio Shack is a ham.  :) 
Anyway, the end-fed zepp is Easy to put up in the woods. Easy to carry. Once I decide on a QRP rig it will probably be the end-fed Zepp antenna for me once again like my novice days. There certainly is nothing wrong with the 1/2 wave dipole, it just takes more room to string it up. That fiberglass pole that David is running is a great idea for a center point to hang a dipole on. The pole would be pretty Light in weight, I reckon. Caution: If you use open wire ladder line like I did in my earlier days, DO NOT touch the wire while transmitting. Don't ask why I know this.   :)  but there are such things as scars.

KK0G

Quote from: gil on September 08, 2013, 07:04:27 pm
QuoteTheir weak point is that the RF energy is carried outside the line and not confined within the shield: this means that they interact with every object they encounter and their path must be carefully planned.


My issue would be getting the line inside the house. Right now I use RG-174 and it goes through an aluminum window frame where it makes two 90deg bends. Very bad, I know... Due to the aluminum, could I route open line the same way, or would I be better off going through the wall? I do have two 50-Ohm 8" barrel plugs that I could use to turn the balanced line into coax, but what would I need to do and would that even be  good idea? The open line would have to turn into a short coax somewhere, right? What if the tuner is inside the radio?

Gil.



This is one of those things I was referring to that while true is blown way out of proportion. The twin lead leaving my tuner passes all kinds of metal objects including several other feed lines, accessory cabling, power cords, metal desk, steel mast, etc. on it's way up to my dipole, in several places it's in direct contact with them. Is there interaction with these things? No doubt there is but it's never been anywhere near enough to cause any problems.


As far as feeding the feedline goes, the long answer is it depends on what antenna you're trying to feed. The short and simple answer is get a cheap MFJ tuner with a built in 4:1 balun.
"Those who would give up essential liberty to purchase temporary safety, deserve neither liberty nor safety" - Benjamin Franklin

KK0G

IZ2UUF

Quote from: KC9TNH on September 08, 2013, 07:33:37 pm
Perhaps Davide can speak some more to that or KK0G on how that gets handled for the fixed location.


I have a big dipole at home fed with an home made ladder line. The ladder line enters a wooden window through two little holes in the frame. On the inside, immediately nearby the window, there is a tuner with "balanced" ports (internally connected to a balun); then regular coax goes to the radio.
I totally agree with KK0G about what many OMs think about twin-lead and me too would never give away my ladder line fed dipole.
It is absolutely true that twin-lead is not easy to deploy: my apartment in Milan covers 8th and 9th floor, which is the roof: so my radio station is literally on the roof, which is an ideal position. In most other cases, when line has to run inside walls, using a twin-lead line is very hard, if not impossible.

However, there are other ways to take advantage of twin lead lines. For example, one could install a cheap automatic tuner well protected in the attic or near the window and use twin-lead from the anntenna to the tuner; then normal coax to run to the station. This solution has the same performances of a mast-mounted tuner but cheaper.

Internal ATUs and twin-lead - I read a question about using internal tuners. In theory, a 1:1 or 4:1 balun at the end of the twin-lead would do: tuners with "balanced" entry are normal tuners with a 4:1 balun in them. However, there are two issues with internal tuners:
1) many of the tune only in TX: this is ok for almost resonant antennae, but these random antennae are completely deaf if not tuned;
2) often their tuning range is too restricted: they are designed to tune antennae slightly out of the resonance point, not random wires.

Balun - Normal tuners have this setup: twin-lead -> 4:1 balun -> tuner -> coax -> radio. This is the only option for this kind of multi purpose tuners. However I think that the best setup would be: twin-lead -> tuner (ungrounded) -> 1:1 balun -> coax -> radio. This is because in this way the balun (that can be a current choke) works without SWR, reducing its power dissipation.
In QRP/P I use this setup: twin-lead -> tuner -> very short coax -> radio. The tuner/coax/radio assembly is so short (compared to wavelength) that doesn't unbalance the line.

73 de Davide IZ2UUF

IZ2UUF

Quote from: gil on September 08, 2013, 07:04:27 pm
My issue would be getting the line inside the house. Right now I use RG-174 and it goes through an aluminum window frame where it makes two 90deg bends. Very bad, I know... Due to the aluminum, could I route open line the same way, or would I be better off going through the wall? I do have two 50-Ohm 8" barrel plugs that I could use to turn the balanced line into coax, but what would I need to do and would that even be  good idea? The open line would have to turn into a short coax somewhere, right? What if the tuner is inside the radio?


The best solution is probably to run the line outside keeping it reasonably far from everything. Then entering the house somewhere and putting an automatic tuner right there; then regular coax goes to the radio.
In theory the line could enter aluminum or other conducting objects as long as they are symmetric to the line: if the field disturbance is symmetric, it does not unbalance the line.

As I wrote in my previous post, most radio tuners do not have enough tuning range to tune this kind of mismatch. Also, many of them tune only when transmitting.

Davide

IZ2UUF

Quote from: gil on September 08, 2013, 08:00:07 pm
Depends on what you call a "commonly used QRP antenna..." If you mean a Buddistick or something similar, sure, though they do work. I use end-feds, and they are as god as a dipole. In fact they are dipoles of a sort, end-fed dipoles. Performance is really a matter of size (that's what she says ;-), no escaping that. The only exception might be the magnetic loop, but you get extremely narrow bandwidth as a price.


Gil

I totally agree with that, except that it is matter of size... and height.
If you are working 20m or higher bands, you can afford a vertical end-fed half-wave antenna, whose performances are more or less the same of a vertical center-fed dipole. The problem with it is that is too near too ground, which causes a great attenuation.
This is the horizontal dipole I've shown and an end-fed 1/2 wave antenna on 20m, both on the same type of ground and 10m pole:



As you can see, the horizontal dipole gains 6 dB more than the vertical dipole; ok, the horizontal dipole is directional, but in its nulls has almost the same gain of the vertical. I mean, vertical 1/2 wave is not bad at all, but the high horizontal dipole seems to be significantly better.

If we go down to 40m, the situation becomes dramatic. The vertical 10m pole is 1/4 wave and behaves as a Marconian antenna: the ground now has a much greater influence:


I tried to center-feed a 10m pole on 40m to make it behave as a shortened Hertzian dipole using 300-ohm twin-lead up to half-way the pole and then continuing with a single conductor. It is a zepp-like feeding: it works, with performaces similar to the 1/4 wave with several radials, but nothing comparable to the dipole.

73 de Davide IZ2UUF

KC9TNH

Quote from: Archangel320420 on September 08, 2013, 09:20:18 pmJust don't bother telling them why you want it. You will be in for a whole lot of nonsense talk about antenna installations and why you should buy coax line instead, unless your guy at Radio Shack is a ham.  :)

LOL, true. Plus they want to sell you a phone plan. I got some (in brown, for the fashion conscious) to cut up for some roll-up J-poles for 2m from an old TV repair fellow who was closing the business. The best source I've found is a smaller-town hardware store (a true one) who stock this stuff on reels with the rest of their wire.  Many folks in rural areas still run this to an aerial on an old tower, bolted to their old house, going to their old converter box - because they don't have cable TV.  So it's stocked by any hardware or farm supply store worthy of the name. They unspool it & measure against the dimension of the floor tiles. And it's cheaper than Radio Schtick. I seem to recall there are even little center-insulators sold somewhere with cutouts to allow for strain relief of twin-lead as the feedline, although they could certainly be made.

I think I'll get that SOTA-type dipole I mentioned up to the hunting property and see how it does. The attraction for me at QRP levels is that twin-lead helps with an issue that downsizing coax doesn't - loss, in a system where every milliwatt is precious.

KC3AOL

Quote from: gil on September 04, 2013, 09:08:37 pm
I'm still planning a 20m magnetic loop, that will be portable...

Gil.

If you're going to DIY a magnetic loop antenna, you could also DIY the variable capacitor...
http://www.alexloop.com/artigo21.html

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