Perhaps goofy antenna idea

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Jock Elliott
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Perhaps goofy antenna idea

Post by Jock Elliott »

On the Radio Reference forum, I asked a question about scanner antennas, one of the guys there kept nudging me to try the off-center-fed dipole -- https://wiki.radioreference.com/index.p ... Fed_Dipole -- and, with the help of a ham friend, I built one, and it works very well for general-purpose scanner use.

So, would it make any sense to build a scaled-up HF version of the off-center fed dipole? I have a 24-foot section of my house where I could hang one under a plastic gutter.

If so, what might be the length of the "arms"?

Cheers, Jock
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Andrew (grayhat)
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Re: Perhaps goofy antenna idea

Post by Andrew (grayhat) »

Jock Elliott wrote: Mon Sep 12, 2022 10:33 am On the Radio Reference forum, I asked a question about scanner antennas, one of the guys there kept nudging me to try the off-center-fed dipole -- https://wiki.radioreference.com/index.p ... Fed_Dipole -- and, with the help of a ham friend, I built one, and it works very well for general-purpose scanner use.

So, would it make any sense to build a scaled-up HF version of the off-center fed dipole? I have a 24-foot section of my house where I could hang one under a plastic gutter.

If so, what might be the length of the "arms"?

Cheers, Jock
Hi Jock, I'm puzzled at why they suggested you the OCFD, see... that antenna was born as a multiband one, in short, if one takes a dipole cut to resonance on (say) the 80m band (3.5MHz), feeding such a dipole at the center would result in an impedance around 60 Ohms, but if we then try using that dipole on (say) 14MHz the impedance will raise to something around 3000 Ohms, making it unusable on that band, but if we then move the feedpoint from center, we'll find that there's a "sweet spot" where, on harmonically related bands, the dipole present an impedance around 200 Ohms, this means that, by moving the dipole feedpoint off center and using a 4:1 transformer (BalUn) we'll be able to use the resulting "off center fed dipole" on multiple (not ALL !) bands (see here for further infos, but you can find more on the internet)

Now, while for reception only the impedance matching becomes somewhat "secondary", using an OCFD as a scanner antenna may make sense if one is just interested in some harmonically related bands, otherwise there are other antennas which offer better matching and good performance, it mainly depends from the range of frequency one is interested in and from the available space to install an antenna

Getting back to the dipole, a non-resonant OCFD makes little sense, also since being non-resonant, moving the feedpoint offcenter makes really little sense, since we won't have the impedance matching advantage; now, as for picking an antenna, which frequency range/ranges are you interested into ?

Also, just to give you an idea, your 24ft section may allow to fit a linear loaded dipole, in such a case you'd have two equal length arms made using 3 conductors cable with the wires connected in series, that would give you a total of 72ft of wire "in the air" (ok, not exactly that given the interactions between the wires), add a BalUn at the feedpoint, an LNA covering the desired frequency range and you'll have a pretty nice RX antenna
13dka
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Re: Perhaps goofy antenna idea

Post by 13dka »

I'm by no means an antenna expert but in addition to what Andrew said, AFAIK OCF dipoles are trading symmetry for the simple match, making them potentially more vulnerable to near-field noise pickup (which is something I experienced several years ago when I replaced an OCF dipole I made to save on coax with a symmetric dipole). Also, horizontal antennas need to be pretty high (more than 30') to have a good DX performance on frequencies <14 MHz.
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Jock Elliott
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Re: Perhaps goofy antenna idea

Post by Jock Elliott »

Andrew,

Thanks for the info.

A couple of questions:

1. What would be the downside of feeding the signal from the dipole to an MFJ 1045C (which I already own), where I could control the level of amplification, instead of the LNA at the balun?

2. I was thinking of using phone wire, which has four insulated conductors inside of it, for the linear-loaded dipole. Would connecting the four conductors be even better . . . or are three conductors optimal?

13kda,

Unfortunately, maximum height is going to be about 10-12 feet off the ground.

Cheers, Jock
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Andrew (grayhat)
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Re: Perhaps goofy antenna idea

Post by Andrew (grayhat) »

Jock Elliott wrote: Tue Sep 13, 2022 10:23 am Andrew,

Thanks for the info.
Hi Jock, you're welcome !
1. What would be the downside of feeding the signal from the dipole to an MFJ 1045C (which I already own), where I could control the level of amplification, instead of the LNA at the balun?
having the preamp directly at (near) the feedpoint will minimize losses (coax) and avoid that the preamp could amplify whatever common mode noise picked up by the coax feedline; the ideal setup would be having a tuned preamp right at the feedpoint, since that way the preamp would only "see" the frequency range "filtered" by the tuned circuit, this would also help avoiding preamp overloading due to nearby off-band transmitters; if that isn't possible, the second best choice imHo would be placing the preamp at the feedpoint and a passive (w/o preamp) preselector before the receiver
2. I was thinking of using phone wire, which has four insulated conductors inside of it, for the linear-loaded dipole. Would connecting the four conductors be even better . . . or are three conductors optimal?
you may use four or more conductors, the "standard" setup uses three, but four may be ok, just ensure to distance the two arms, I mean, leave the two conductors going to the balun a bit longer (say 4 inches each) so that the linear loaded arms will be spaced a bit; as a note, ad advantage of using 3 (or an odd number of) conductors is that the "free" end of the last conductors will be at the end of the arms, that could allow to connect to those two pieces of single wire "dangling" down, this won't be possible if using an even number of conductors since the "ends" would then be near the feedpoint

Oh, and... 10t from ground means that the antenna will be very low with respect to the wavelength(s) you're willing to receive, so its main lobes will be oriented toward the sky, in this case, a vertical would be a better idea, since even if placed low its takeoff angle will remain low with respect to the horizon
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Jock Elliott
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Re: Perhaps goofy antenna idea

Post by Jock Elliott »

Andrew,

Thanks again!

A couple of additional questions:

- I was thinking of using phone line wire, which has four conductors. Do I presume correctly that there is no harm in leaving one conductor unused?

- Any verticals that you could recommend?
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Andrew (grayhat)
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Re: Perhaps goofy antenna idea

Post by Andrew (grayhat) »

Jock Elliott wrote: Tue Sep 13, 2022 12:19 pm - I was thinking of using phone line wire, which has four conductors. Do I presume correctly that there is no harm in leaving one conductor unused?
Instead of leaving it alone, connect it in parallel with one of the others, that way you'll still have 3 conductors but one of them will be "fatter"
- Any verticals that you could recommend?
well, in your case, given that you have some horizontal space and some vertical one, an idea may be putting together a T antenna, basically you'll have two horizontal "arms" (the top capacitive hat) connected together and at that point, connected to a third wire coming down vertically to (near) ground (the below is a large T antenna) used for MW transmissions

https://en.wikipedia.org/wiki/T-antenna ... WOR-AM.jpg

Image

the antenna will then be fed between the vertical wire and a short wire connected to a ground stake using a transformer

https://en.wikipedia.org/wiki/T-antenna

notice that the "real" antenna is the vertical one, while the horizontal section act as a capacitive hats which has the result of making the antenna "longer" (at RF) like other loading methods
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Andrew (grayhat)
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Re: Perhaps goofy antenna idea

Post by Andrew (grayhat) »

just in case, modeled a T-antenna using the sizes you gave (7m total for the horz wire - 2x3.5m arms, 3m for the vert one) for the horizontal and vertical portions and here's what to expect from it

Image

as you can see, the pattern is omnidirectional with pretty low takeoff angle, the gain on 1.8 MHz is around 5dBi (if using ground radials, much less with a ground stake) which isn't bad and can always be boosted with a preamp/preselector; notice that the takeoff angle will slowly raise going up in frequency, but the lobe will remain wide enough to still allow to pick up signals at lower angles
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Andrew (grayhat)
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Re: Perhaps goofy antenna idea

Post by Andrew (grayhat) »

just in case, here are the graphs related to the T antenna used with a ground stake

Image

as you can see, the omnidirectional pattern is preserved over the whole shown range (and below it) while the gain is pretty low, but can be easily compensated using a preamp, all in all we're chasing SNR not gain here; also, the radiation lobe is quite good for DX and medium distance too but not for NVIS, but that shouldn't be an issue, I suppose
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Jock Elliott
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Re: Perhaps goofy antenna idea

Post by Jock Elliott »

Andrew,

That looks great! Thanks for the hard work.

I presume the coax to the antenna counts as the vertical part of the T?
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