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Author Topic: " Feedline " Antenna - "450 " Ohms  (Read 357 times)

Offline NQC

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" Feedline " Antenna - "450 " Ohms
« on: June 08, 2021, 1531 UTC »
Hey All,

I want to try an antenna that would be for temporary use  in areas of possible power line noise.

I envision running 2  insulated  multi stranded   wires  ( AWG size TBA , aluminum vs copper  composition TBA ) ,
 air spaced at 1"   to  "simulate "  foam twin lead in  the  ball park  300 to 450 ohm  range     .
Conductors are  shorted at the elevated end.

I am shooting to build a supposed  folded  vertical "quarter" wave/ "low noise " design  for  1 Mhz. It's a bit unclear to me what length is "ideal" and  what  length actually represents  a TRUE quarter wave when it is  folded .

FWIW, At 1 Mhz a quarter wave MONOpole  would be 234 ft (  for now I am disregarding any "Earth" side considerations such as 45 degree  drooping  radials, buried "spoke" radials, etc). 234 feet is  WAAAAY  TOOOOO TALL !

But I AM thinking about a possibly  alleged "desired" electrical  length of 117 ft.

117 ft is also a "practical " length to actually be able to work with.  I am not sure  whether (2X) 117 ft would  still   only "represent" an 1/8 monopole in reference to the wire run's total  vertical height -OR- whether it acts as a "true" folded 1/4 wave, having one leg running up to full height and one leg running back down to "lawn level" (117 ft "plus" 117 ft).

BTW, I am sure that there is no way I can hang it "fully" vertical. I'd be LUCKY to get it up 40 or 60 ft. So there would probably be more than  HALF of the antenna's  total length arcing over and running fairly low  horizontal to the  lawn (like a kite string). I can probably get    a tie off on a low tree branch at 6 ft or so , then down to radio. Implications ?

I also wonder if  giving this "twin  lead " a few twists is advantageous. I have seen this done when twin lead is actually used to FEED  a "real" antenna (  ie a horizontal dipole ).

For  the  receiver, I'd make  up a 175 or 220 uH (wound with #28) ferrite bar with maybe 4 or 5 "taps" mostly bunched up at the cold end- hopefully one of which winds up  at a ball park  300 ohms or greater (otherwise put, guesswork).
 A  properly spec'd ceramic var cap would be across full coil,so  the project would end up being a twin lead tuned  ferrite  auto transformer air /magnetically coupled to the receiver's internal  ferrite.

I know that design specs  and "rules" are much less important on receive antennas as  opposed to transmit antennas.Also  the "characteristic response" is going to  vary as one moves up or down in frequency away from the "design reference" of 1 Mhz. There is one school of thought that says "Just try it and see how it works". The problem is that I don't have NEARLY as much to time play radio as I would like, so I'd like to get a bit of  back round , input and "predictions" before I forge ahead.

Also, it  can be a bit of  learning experience and  if nothing else, a  theoretical exercise  on how antennas work.

ANY help GREATLY appreciated,

de N1NQC 

« Last Edit: June 08, 2021, 1543 UTC by NQC »
Station main receiver : Bed springs to  blue razor blade detector to 2000 ohm cans to steam  radiator. Grid FN 42

Offline Stretchyman

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #1 on: June 08, 2021, 2143 UTC »
Using 450R feeder, you can drill holes thru the plastic spacing and thread a 3rd wire thru. Join the top to one side of the feeder and then the bottom to the other. The antenna will go up, down and up again, kinda like double linear loading. Works better than using a loading coil obvs with a fairly narrow B/W. Pair of raised radials with similar loading completes the antenna. You'll need a conjugate match at the base obvs..

Str.
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Offline NQC

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #2 on: June 09, 2021, 1120 UTC »
Hey Stretchy,

Interesting ideas to think about.

Thanks for the info,

de NQC
Station main receiver : Bed springs to  blue razor blade detector to 2000 ohm cans to steam  radiator. Grid FN 42

Offline RobRich

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #3 on: June 09, 2021, 1346 UTC »
If you are seeking to diminish high power line noise, a large omnidirectional antenna regardless of resonance quite likely will make the problem (much) worse instead of better.

Recommendation? Consider building a capacitor-tuned mediumwave vertical loop. Rotate it to peak a station and/or help null the local noise.
Tampa, FL USA | US Map Grid EL88
Airspy HF+ Discovery | Kenwood R-600 + R-2000 | 2x RTL-SDR V3 + NE602 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals | 2x PA0NHC MiniWhips

Offline Stretchyman

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #4 on: June 09, 2021, 1509 UTC »
How true!

Yes avoid verticals for Rx, best for TX.

Loop would be good if tuned for Hi Q.

Str.
« Last Edit: June 09, 2021, 1512 UTC by Stretchyman »
'It's better to give than receive' so why RX when you can TX!

         15W and100W models available.

                   Buy one from me, NOW!


                                              ;)

Offline NQC

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #5 on: June 09, 2021, 2100 UTC »
Hey All,

The spot were I would set up may not be SUPER  noisy, if I gave  the wrong impression on that I apologize.

The vertical folded quarter wave is supposed to be a “ low noise”  (“magnetic “ ?) antenna.

One of my goals ,other than a reduction of some possible noise, is to have an antenna that has some decent capture area.

FWIW I am also in the process of building a BIG octo loop around 5 ft diameter and (at least temporarily) wound with 660/46 litz. The var cap will be a 4 gang /500 pf ceramic  wired “wiper less” to eliminate wiper loss.

After this post it’s down to the shop to work on the loop.

So I have a couple of things to eventually try ( other that the Select -A - Tenna loop, which is very good but has limitations due to it’s size).

Again, thanks for the replies.


de K
« Last Edit: June 10, 2021, 0029 UTC by NQC »
Station main receiver : Bed springs to  blue razor blade detector to 2000 ohm cans to steam  radiator. Grid FN 42

Offline RobRich

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #6 on: June 10, 2021, 0630 UTC »
Outside of some very complex equations or modelling software, I recommend simply assuming both electric (E) and magnetic (H) fields are going to have similar influences within the reactive near field of the antenna. Even tossing linear loading aside, an 117' vertical would have an approximate reactive near field distance of around 30 feet. An actual 234' wire is going to be more like 70'.

Note your antenna is still subject to Kirchhoff's first law. A 234' wire or loaded approximation thereof without the other "half" of the antenna is going to be an 1/4-wave longwire around 1MHz, or alternatively, an "end-fed halfwave dipole"* around 2MHz if fed using a high ratio transformer. ;) So ideally you will need some type of RF ground for the proposed quarter-wave element, such as multiple detuned radials on the ground or quarter-wave radials elevated above ground.

Otherwise get you can end feed a wire to get around the radial issue by treating it as an EFHW dipole, but you will need to *double* the wire length. Use a 49:1 or 64:1 transformer at the feedpoint. One side to the antenna and one side to a simple RF ground; just a few ground radials or even a 8' ground rod should suffice. If you do not want to even bother with those, place a 1:1 current balun as a RF choke a few feet down the coax feedline. Common mode coupling is not my first choice for a RF ground, but we are talking about receiving instead of transmitting here.

Regardless of the RF ground path, remember Kirchhoff still applies. An EFHW is basically an extremely offset fed dipole fed near a voltage node. There is going to be coupling to something to complete the antenna, thus often the feedline ends up being the other part of the antenna.

About twinlead, twists every few feet *might* help reduce common mode, but only as a feedline with differential currents present. As to the described single linear element of antenna system, there is no differential current on the single element, so it is unlikely to do anything except maybe induce some type of helical loading into the mix.

Long story short? Your described loop is a much better idea IMHO. Let us know how it works for ya'. :)
« Last Edit: June 10, 2021, 1310 UTC by RobRich »
Tampa, FL USA | US Map Grid EL88
Airspy HF+ Discovery | Kenwood R-600 + R-2000 | 2x RTL-SDR V3 + NE602 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals | 2x PA0NHC MiniWhips

Offline NQC

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #7 on: June 10, 2021, 1208 UTC »
Hey Rob,

One of the problems I have is that I have no coax fed receivers left,the R- 70 and R-71a are long gone.

So everything has to be inductively coupled to my ferrite rigs ( ie the ATS 803a,etc)

Yes , the loop hopefully will work out.

But again, I was also thinking of trying for something larger, especially for long  dx frequencies that have no qrm or qrn issues.

I really have nothing to lose by simply trying it, the investment in materials is fairly small.If it doesn’t work out , it’s no big thing.

If nothing else, I am learning some new things as I go along.

I have some good “ tests” that I can use during the day .Ultra weak WINS 1010 or WMCA  570 from NYC , both of which are 20 Kc away from super strong locals
WEEI on 590 and WBZ on 1030.

I see how it works out.

My thanks for the reply.

K
Station main receiver : Bed springs to  blue razor blade detector to 2000 ohm cans to steam  radiator. Grid FN 42

Offline RobRich

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #8 on: June 10, 2021, 1308 UTC »
I have coupled various antennas to portables for mediumwave with a Tecsun AN200. I fed the larger antenna into the AN200 1/8" external antenna port, then placed it near a portable's internal loopstick as usual. Q tuning was not as sharp, but it otherwise worked okay, at least with the various antennas I have tried.

You could do similar with your proposed cap-tuned, multi-tap ferrite bar coupler. :)

I would start with a whatever length long wire as vertical or inverted-l, though in this particular case it probably would be a good idea to actually stay away from 1/2 wavelength if end feeding the wire directly into the ferrite coupler due to very high impedance.

I suspect you might find an 1/8 wave or even shorter workable for improving mediumwave groundwave reception with a typical portable.
Tampa, FL USA | US Map Grid EL88
Airspy HF+ Discovery | Kenwood R-600 + R-2000 | 2x RTL-SDR V3 + NE602 | Si-Tex 200 | Soft66LC4 | Yaesu FRG-7
148' "Shielded" Loop-on-Ground | 18' End-Fed Vertical | 9' + 31' Verticals | 2x PA0NHC MiniWhips

Offline NQC

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Re: " Feedline " Antenna - "450 " Ohms
« Reply #9 on: June 10, 2021, 1954 UTC »
Hey Rob.

I had this set up many years ago:

A VERY high Q 660/46 coil was on a 4 gang ceramic . My 100 ft roof top inverted L on directly hot end, no ground
( ground killed Q) .

When coupled to a ferrite radio the tank was SO hot that you could blow by a station very easily, even with a 6:1 ball drive.

The audio was pinched SO tight, it made my superhet sound like a regen.

The Q was simply ridiculous.

I may not get the same results with my future pursuits, but MAN, what a set up THAT was !

Kind of nice that “ less may be better “ ( 1/8 wave or less) for a quickie set up /take down.

K
« Last Edit: June 10, 2021, 1959 UTC by NQC »
Station main receiver : Bed springs to  blue razor blade detector to 2000 ohm cans to steam  radiator. Grid FN 42