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Fixed Phasers For Pairs of Antennas
Dave Aichelman IRCA Convention 2019
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I blame it all on a trip to Kauai back in 2012…
I needed a small portable antenna for DXing and developed the “tent pole loop” for the unwary TSA agent, some, ah, “camping equipment”:
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for the DXer, a quickly assembled 7. 5’x7
for the DXer, a quickly assembled 7.5’x7.5’ loop antenna, or even two for a phased array:
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I returned with great captures and a favorable impression of small antennas
however, a single small loop isn’t a DKAZ, which has marvellous directivity ---and directivity can be important, particularly on the coast assume everyone is familiar with the DKaz antenna? plenty of signal from “front” two quadrants (of the compass?) little from the “back” two
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even small DKAZ antennas have that directivity, and I’ve used ones as small as 11x64’ like this one up on Mingus Mountain, AZ but that small DKAZ can still have a lot of ‘presence’ at a DXpedition site and can be a lot of work to setup for a short DX session …by 4dB though not quite as directional (this is using EZNEC models…no gain for either) (& a 21 x 88 DKaz is 10dB more sensitive…)
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plus….DKAZ antennas are quite insensitive at the low end of the MW band, and small ones are even more insensitive for example, a 7.5’ x 7.5’ “tent pole loop” configured as a “Flag” is actually more sensitive to DX at 600kHz than is an 11 x 64’ DKAZ!
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----but could two small loops offer competition to a DKAZ?
I knew that a small loop could be used with preamplification to hear DX, both empirically, and from Dallas Lankford’s theoretical work. Wellbrook’s preamplifiers offer plenty of gain for use with small loops so….even small loops might provide building blocks for a useful phased array
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----in fact, Dallas Lankford has also done a great deal of work on “fixed” phasing units, and spaced antennas
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his work is detailed and complex, so I simplified it for my own purposes
I was able to create a passive and low loss phasing unit, to be used with “tent pole loops” that is great for travel and rapid setup
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---my design combines two loop antennas, a small homemade module with a commercial ‘Magic-T’ bassplitter/combiner this could be divided up to show the loops, then the module, and finally the combiner XXXX….or just use pointer to various components
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---my design combines two loop antennas, a small homemade fixed phaser module a commercial ‘Magic-T’ based splitter/combiner this could be divided up to show the loops, then the module, and finally the combiner XXXX….or just use pointer to various components
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---my design combines two loop antennas, a small homemade fixed phaser module with a commercial ‘Magic-T’ based splitter/combiner this could be divided up to show the loops, then the module, and finally the combiner XXXX….or just use pointer to various components
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the heart of the phasing unit
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inside the phase shifter
derived from Dallas’ work. My handout includes phase shifters appropriate for 50 and 25 feet as well.
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some capacitors and an inductor
a Lankford-designed phase shifter specific for 100’ separation of antennas and 50 ohm cable and components the component values derived from Dallas Lankford’s work. simply… some capacitors and an inductor
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further details --These designs are for arrays that use antennas with 50 ohm output --The two feed-lines should be 50 ohm coax , the same length from each antenna to the phasing unit / combiner, and use the same vendor and type of cable.
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further details a fixed phaser module is designed for a pair of antennas separated by one specific distance for example, I have built modules for antenna separation of 100, 50, and 25 feet described in the accompanying handout
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further details ---I’ve used pairs of bidirectional loops as well as Flag pairs, but pairs of whips or active dipoles can also be used with this phaser. ---The antennas should match each other in type, size and termination and be placed at the designed separation distance, inline with each other.
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further details one of the antennas may need to be physically phase reversed relative to the other, in addition to the shifting that takes place in my phaser with loops and Flags, reversing the phase simply involves reversing one antenna’s leads to the preamplifier or matchbox ---- if nulls are not found with the initial setup, reverse one antenna’s leads
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Note!! this is a fixed phaser and can offer only average nulls across MW an adjustable phaser will always have the ability to null individual stations more deeply than any fixed phaser.
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Observations small loops deliver lower signal levels than larger ones
doubling or halfing the circumference gives +/- 10 to 12 db change in sensitivity a large 20’ x 140’ DKAZ has ’ circumference loop elements home 12’x32’ loop/flag array has 72’ circumference loop elements, about half the size phased loops with small separation between loops deliver lower signal level than those with larger separation between the loops
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Observations I’ve specified 100’ separation between the loops here, as that will always produce more signal than 50’ or 25’ separation for AM BCB the greater the separation between the loops, the greater the signal output, but separation greater than about 130' offers diminishing returns in improved signal output vs nulling across MW 6dB for going from 50’ to 100’ separation with appropriate change in phasing unit
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Observations The phaser component values and antenna spacing are far more tolerant of variation than I expected, however it is best to use 5% tolerance components. Mismatched antennas and feed-lines lengths will cause far more trouble, as a fixed phaser cannot be adjusted to compensate
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Observations phased loop antennas deliver lower target DX strength than does a single loop antenna, BUT phased loop antennas provide broadband nulls in the “back 180”, so that DX will suffer less interference from behind the array
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Observations phased flag antennas deliver lower target DX strength than a similar sized loop antenna array, BUT they can provide even better broadband nulls in the “back 180” of the pattern
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loop antenna phased array pattern:
Observations loop antenna phased array pattern:
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Flag antenna phased array pattern:
Observations Flag antenna phased array pattern:
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and…for reference, a DKAZ pattern:
Observations and…for reference, a DKAZ pattern:
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Observations Observations Loop array Flag array DKAZ
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Observations I see 15-25dB back end nulls across the band with pairs of bidirectional loops and 20-35dB nulls with pairs of Flag style loops. Even though Flag arrays “seem better” because of better backside nulls and low- angle forward reception… phased bidirectional loops do provide more target signal level
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Observations In addition, loop arrays have more high angle reception in the forward direction, so the ‘dawn enhancement effect’ may be more pronounced on ocean paths using a loop array . 24 Flag vs 31 degrees at 1MHz Phased loop maximum arrival angle = 31°
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Observations In addition, loop arrays have more high angle reception in the forward direction, so ‘dawn enhancement effect’ may be more pronounced on ocean paths using a loop array rather than a Flag array . 24 Flag vs 31 degrees at 1MHz Phased Flag array maximum arrival angle = 24°
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Observations phased arrays and the DKAZ all suffer from a decrease in signal level at the lower end of the medium wave band this roll off is most pronounced with the DKAZ, followed by phased Flags, followed by phased bi-directional loops
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Conclusions we started by asking if phased “tent pole loops” could compete with a DKAZ although I’ve used different arrays, and DKAZ antennas, I’ll compare here the 100’ separation “tent pole loop” array with a x 88’ DKAZ, 30’ vs 105’ loop element circumference
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Conclusions similar requirement for ~100’ end to end, BUT
TP array takes up less volume and has more separation between elements loop array DKaz
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Conclusions using 2 small loops with a fixed phaser, makes for a quick easy setup in a small amount of space. plus, they can provide a better nulling pattern than a single flag antenna, and match a DKAZ pattern if using a flag array can they compete with a DKAZ?
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Conclusions the proof is in the listening
I have used phased “tent pole loop” arrays in Grants Pass, Oregon, 55 miles from the coast Under good conditions, sensitivity was good enough to hear the low power Aussie ethnic stations on the extended band, as well as KHMB Half Moon Bay 1 watt 'community' broadcast station at 350 mile distance
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Conclusions So…. Under good conditions, a “tent pole loop array” can be used for TP DX the “tent pole loop array” can work well for domestic DX and East Coast TA DX a loop array 2X the size of TP loops, at 100’ separation, starts to get competitive with a smaller 21’ x 88’ DKAZ array for TP DX
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Conclusions the intent of this talk is to spark interest in trying new antennas and phasing in general antenna experimentation doesn’t always have to be too complex, or take up too much space Bidirectional and Flag arrays can be smaller than a DKAZ because they have fewer losses and more space between loop elements, hence more signal for their size
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There is a handout here with further details for building fixed phasers to use with small loops, whips and active dipoles There is another handout of how I build DKAZ and other loop antennas. I look at the DKAZ differently than most if you need help.
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