HFU HF Underground
Technical Topics => Equipment => Topic started by: ultravista on September 20, 2017, 1333 UTC
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I have an opportunity to string-up either a long wire or dipole for HF and Short Wave listening.
Not interested in freqs. above 20 MHZ.
I can get about 140 feet of wire up in the air, either a long 140 run or two 70 foot runs as a dipole.
As this is receive only, would a dipole offer any advantages over an end fed wire?
Instead end fed, what about a center fed long wire?
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I have an opportunity to string-up either a long wire or dipole for HF and Short Wave listening.
Not interested in freqs. above 20 MHZ.
I can get about 140 feet of wire up in the air, either a long 140 run or two 70 foot runs as a dipole.
As this is receive only, would a dipole offer any advantages over an end fed wire?
Are there any bands you are particularly interested in? If so, a dipole would probably be the better choice of the two. And a folded dipole will likely be less noisy than a regular dipole. It is usually built out of 300 ohm ladder line, and fed with a 4:1 balun, and you can then use RG6 coax to feed it.
Actually I think your best choice would instead be to construct something like a T2FD antenna, especially if you can slope the antenna (one end much higher than the other). A 140 ft long T2FD (potentially even longer depending on the angle of slope) would be a very good performer for you, and cover pretty much the entire HF band. I had a T2FD years ago (before I built the sky loop) and it worked very well.
Instead end fed, what about a center fed long wire?
Short answer - that won't work well.
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Palomar Engineering (http://palomar-engineers.com/tech-support/tech-topics/off-center-fed-ocf-antennas (http://palomar-engineers.com/tech-support/tech-topics/off-center-fed-ocf-antennas)) used to sell an off center fed multiband dipole (p/n: PAL-OCF8010) that runs about 125 ft. long.
I have one at about 65 ft high and it works very, very well. It is also used for ham band TX as well. They still sell the parts to make one but they may also sell them outside of their online catalog if you ask for it.
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A t2fd is more complex in construction and mounting compared to a cf or ocf dipole, but the t2fd's reduced noise and broadbandedness are compelling.
http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html
That being said, a ocf dipole can ease construction and erection as it has less fiddly bits than a t2fd with its terminating load and twice the amount of wire in the air, as well as random placement of the feed point, possibly allowing you to use that existing coax you thought was going to be too short, it also reduces sag as the feed point's not in the center. A cf dipole will have harmonically related periodicity that is base band related, a ocf dipole will have periodicity about every other MHz from the base band.
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My goal is below 20 MHZ, shortwave and below.
Thank you all for the guidance.
I know there are optimum lengths and lengths to avoid for transmitting - just wondering if there is a reciprocal for receiving.
For the T2FD, considering it is RX only, can I use 50 or 75 ohm coax?
With a T2FD for RX only, do I need a terminating resistor? What is the terminating resistor for?
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Antenna length isn't as critical for receiving as it is for transmitting.
You may notice a small boost, but you wouldn't notice more than a db or two at best.
Guys use 100 foot wires to DX the MW/AM band, and that is not even close to a quarter wavelength at those frequencies. But still, such antennas work well for receiving a wide range of HF and MF frequencies.
Antenna length is more critical when transmitting, and maybe if you're making a beam.
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My goal is below 20 MHZ, shortwave and below.
Thank you all for the guidance.
I know there are optimum lengths and lengths to avoid for transmitting - just wondering if there is a reciprocal for receiving.
For receiving, yes and no - and it depends on the antenna :)
A dipole is resonant on one band (and odd harmonics of that band). It will work elsewhere for receiving, but perhaps not as well.
A beverage, random/long wire, sky loop or T2FD works over a very large range of frequencies/bands.
For the T2FD, considering it is RX only, can I use 50 or 75 ohm coax?
Yes, depending on the balun you select. With a 4:1 balun, 75 ohm, for 50 ohm you'd want something around a 6:1. Ideally. But again as this is receive only, you're more tolerant to impedance mismatches vs transmitting.
With a T2FD for RX only, do I need a terminating resistor? What is the terminating resistor for?
Yes, the terminating resistor is necessary, there's good information about, as well as the T2FD in general, here: http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html (http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html)
While the T2FD is indeed more complex than a dipole, it is going to give you better performance than a dipole or random wire antenna. Significantly better performance. As in the difference between picking up tough DX targets well, or not hearing them at all. Never skimp on your antenna. Also consider a sky loop if you have the room, and necessary trees for support. That's what I use as my primary antenna. And I think my logs speak for themselves ;D
Also, a shameless plug for my universal isolation matching transformer, The Squid, which would work well for a T2FD or sky loop: http://blackcatsystems.com/rf-products/matching-transformer-unun-balun-beverage-longwire-k9ay-flag-ewe-dipole-antenna-shortwave-ham-radio.html
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Over the past 20 years, I’ve built and used the following antennas: T2FD, K9AY, Flag, Ewe, random wires and horizontal dipoles. For me, it’s hard to beat the horizontal dipole’s performance and simplicity if you’re mainly interested in a particular band. Presently the only antennas I have up are dipoles, cut for the bands I primarily DX.
If you don’t have any antenna building experience, try the dipole or longwire first; better yet build them both, and try them out side-by-side and see which you will prefer. Most likely you will find neither one superior to the other, and that they may even complement each other. And that’s when the fun begins, as you keep experimenting, trying to find that one “super antenna”.
For the longwire, I recommend you feed it with coax, through an impedance matching isolation transformer, as this will greatly reduce your noise pickup. Bruce Conti has an excellent tutorial on the construction of these transformers. I’ve built several for DXers using the Rubbermaid baby food container as the housing, and this has made the difference between hearing mostly noise to logging some real DX:
http://www.bamlog.com/diyxfmr.htm (http://www.bamlog.com/diyxfmr.htm)
Another tip for reducing noise is to use RF chokes to sanitize your coax as described here by John Bryant:
http://www.dxing.info/equipment/coax_leadin_bryant.pdf (http://www.dxing.info/equipment/coax_leadin_bryant.pdf)
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A t2fd sans load will be just a folded dipole, and these are noted as rejecting energy at twice their fundamental frequency of operation.
So if you make one say for 80m, it will be poor on 40m, and show peaks and nulls as one goes thru the hf spectrum. What the terminating load in the t2fd does is broadband it, reducing the peakyness of the standard folded dipole. The folded dipole and the t2fd are both very quiet antennas, I had a 10m folded dipole fed with twin lead and a balun, it was so quiet sigs just popped in out of nowhere, you could hardly hear any band noise but keep in mind that band noise in a quiet location on 10m is very low to begin with.
The t2fd may be more quiet as the load transforms some rf into heat and reduces the composite noise plus desired rf signal levels, comparing wire length for wire length. Speaking of wire length, the more wire you have up, the more rf it picks up from a passing wave, simple as that. Compare a dipole for 2m to one for 80m - each having the same "gain" at their frequency of resonance, yet the 2m antenna will not pick up anywhere near the amount of rf that will be available from the 80m antenna. Also, keep in mind that an rx antenna may reradiate 1/4 to 1/3 of all the rf it picks up, and I suspect that this is where linear loading comes into effect as it helps gather up more of that reradiated rf and sends it down the feedline instead of reradiating it back into the air.
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Agreed, folded dipoles are extremely quiet, I have one for 43m (which is also sloped or tilted). Likewise, the sky loop is quiet. In general, any sort of a "loop" antenna where the ends are eventually connected together seems to be much more quiet than a regular dipole antenna. The beverage is also fairly quiet, but again, it's got a termination resistor at the end going to ground, so perhaps it could be considered, crudely in the same category.
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What is the smallest effective Sky Loop? Does it require a transformer or something other than an UNUN?
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While the T2FD is indeed more complex than a dipole, it is going to give you better performance than a dipole or random wire antenna. Significantly better performance.
ChrisSmolinski - for RX only, what T2FD length do you recommend for shortwave & below?
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I just run 310 feet of thin plastic covered wire at 18 cents per metre and have it strung through the tree branches on the property in a "U" shape.
Works very well on all of HF and especially the low wave NDB band and Medium Wave AM band.
Any receiving antenna is only as good as the receiver it plugged in to, another important factor to consider. :D
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While the T2FD is indeed more complex than a dipole, it is going to give you better performance than a dipole or random wire antenna. Significantly better performance.
ChrisSmolinski - for RX only, what T2FD length do you recommend for shortwave & below?
There's a page with the formula here: http://www.hard-core-dx.com/nordicdx/antenna/wire/t2design.html (http://www.hard-core-dx.com/nordicdx/antenna/wire/t2design.html)
For the example on the page, a T2FD for 3300 kHz and up would be 99 ft long and 3 ft wide. (Don't fret over getting it exact to the fractional inch, it is not necessary to be that precise. Obviously you want the width to be more accurate than the length, you are trying to keep the ratio correct). My T2FD was about 125 ft, from memory.
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Right, for our porpoises one could use window line as the folded bits as well as feed line, unless one wanted coax all the way to the feed point. What one should be more precise on is in relation to the resistance value used and the balun ratio but ballpark range is good enough for rx.
Also here's a LoG antenna for your perusal;
http://www.kk5jy.net/LoG/
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There's always the old reliable Windom. You can find the formula for building one on any antenna site and it's been a proven performer for a century. Wire element w/ an offset wire feedline cut to resonance for your preferred frequency. Hook the feedline a simple tuner to match it for wide band listening/ transmitting. It's dirt cheap and it works.
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There's always the old reliable Windom. You can find the formula for building one on any antenna site and it's been a proven performer for a century. Wire element w/ an offset wire feedline cut to resonance for your preferred frequency. Hook the feedline a simple tuner to match it for wide band listening/ transmitting. It's dirt cheap and it works.
I forgot about that antenna. Definitely would be a good, simple antenna for someone to try out. No coax needed.
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With any fed wire antenna one should expect rfi. No one cared about rfi much when the windom was created as everything was tubes and we had nowhere near the amount of rfi emitting devices in the given urban setting - concentrated in the home, but by all means try one and see how goes it.