Delta Loop Feed Model Comparisons

One of the hams on my twit­ter feed, Dan YO3IBW posed a ques­tion ask­ing if he should run a delta loop for 40M. Dan’s anten­nas are sit­u­at­ed on the top of his apart­ment build­ing in Bucharest. The base of his mast is sit­u­at­ed at about 30M above street lev­el, with anten­na fir­ing E/​W. I decid­ed I would mod­el sev­er­al anten­na sce­nar­ios to help illus­trate Dan’s options.

I am a huge fan of delta loops hav­ing had one for 20M in my col­lege days. They are inher­ent­ly qui­et, exhib­it gain over a dipole, are easy to con­struct and are some­what broad-band­ed. There are essen­tial­ly two modes a sim­ple delta loop can oper­ate in: Hor­i­zon­tal­ly-polar­ized and thus like a dipole, and ver­ti­cal­ly-polar­ized and thus like a ver­ti­cal. I have omit­ted dis­cus­sion of the hor­i­zon­tal­ly-mount­ed delta, where each of the three ver­tices of the anten­na are at the same height – this is anoth­er type of anten­na entire­ly not unlike a rhom­bic antenna.

My pre­vi­ous con­fig­u­ra­tion was fed as a SCV, and it was the best ver­ti­cal anten­na I have ever used. I com­pared to an Cushcraft R5 and a Hus­tler BTV5, both of which were nev­er used once I had installed the delta loop.

40M Delta Loop Model

The illus­tra­tion above is a very sim­ple delta loop, equi­lat­er­al tri­an­gle with feed point at the bot­tom. Oth­er feed point options are at the top (for hor­i­zon­tal polar­iza­tion along with bot­tom), and side or cor­ner-fed for ver­ti­cal polar­iza­tion. The loop is made from one-wave­length of wire at the design fre­quen­cy of 7.1, or 42.2M. Each side is ~14M with the ver­tex of the anten­na about 7M high­er than the base.

In hor­i­zon­tal-polar­iza­tion mode, either top of bot­tom-fed at the height of 30M above the street, this anten­na shows a clas­sic figure‑8 of a dipole and has a very low take-off angle.

40M Delta Loop, Horizontally Fed at 30M High
40M Delta Loop, Hor­i­zon­tal­ly Fed at 30M High

Note the oblique angle is approx­i­mate­ly the same as the wire illus­tra­tion above. The Y‑axis is point­ing North, the X‑axis is point­ing East. The max­i­mum gain is broad­side to the wire, essen­tial­ly E/​W, with a take-off angle of about 10 – 20 degrees. At the ele­va­tion angle of 15 degrees (FB for DX), the azimuthal pat­tern is a clas­sic “Fig­ure 8”.

40M Delta Loop, Horizontally Polarized at 30M High
40M Delta Loop, Hor­i­zon­tal­ly Polar­ized at 30M High

Of course, the blob-ish top hat of RF radi­at­ing at a very high angle can­not be avoid­ed due to the height above ground of 34 wave­lengths. The end result of this high-angle blob will be util­i­ty as NVIS (only when ionos­pher­ic FOT will allow) and pos­si­bly a bit more com­mon-mode noise. Nei­ther of these sit­u­a­tions would crit­i­cal­ly affect the low-angle DX per­for­mance of the antenna.

If the delta loop were fed on the side or cor­ner, the angle of radi­a­tion is typ­i­cal­ly at low angles. In this mode the anten­na acts more like a self-con­tained ver­ti­cal anten­na. L. B. Cebik W4RNL left us a lega­cy in his work around SCVs and should be ref­er­enced to learn more about SCV and anten­nas and mod­el­ing in general.

40M Delta Loop, Vertically Polarized at 30M High
40M Delta Loop, Ver­ti­cal­ly Polar­ized at 30M High

A cor­ner or side-fed delta exhibits good low take-off angles in gen­er­al. I this case, due the height above ground, the pri­ma­ry lobe is actu­al­ly at 10 degrees and a sec­ondary lobe is at 40 degrees. The 3dB ver­ti­cal beamwidth is actu­al­ly 47 degrees, mak­ing the take-off angles of this anten­na use­ful for DX and some medi­um range communications.

If this anten­na were actu­al­ly less than 12 wave­length above ground, you would see a more tra­di­tion­al ver­ti­cal take off pat­tern. A gen­er­al rule of thumb is to mount ver­ti­cal anten­nas many mul­ti­ple wave­lengths or less than 12 wave­length above ground. Many wave­lengths above ground pro­duce many well-formed lobes of radi­a­tion. Less than 12 wave­length above ground pro­duces one well-formed lobe of low-angle radi­a­tion. At 34 wave­length above ground, this “ver­ti­cal” has inde­ter­mi­nate for­mu­la­tion of lobes but still a use­ful ver­ti­cal pat­tern in my opinion.

The azimuthal mod­el is more omni­di­rec­tion­al than the bot­tom-fed delta loop.

40M Delta Loop, Vertically Polarized 30M High - 15 Degree Azimuth
40M Delta Loop, Ver­ti­cal­ly Polar­ized 30M High — 15 Degree Azimuth

As ver­ti­cal anten­nas go, this is a good per­former. Con­sid­er how the least favored azimuth is still equal to the best gain of an aver­age ver­ti­cal anten­na. I hope to have a com­par­i­son of ver­ti­cal vs. SCV Delta Loop post­ed in the near future. As men­tioned before, the ver­ti­cal­ly-polar­ized Delta is the best ver­ti­cal anten­na I have used.

Anten­na mod­els are most use­ful when used in com­par­a­tive analy­sis. I have super­im­posed four dif­fer­ent Delta Loop mod­els to eval­u­ate their ver­ti­cal take-off patterns.

Comparative Evaluation of Horizontally and Vertically Polarized Delta Loops
Com­par­a­tive Eval­u­a­tion of Hor­i­zon­tal­ly and Ver­ti­cal­ly Polar­ized Delta Loops

The ele­va­tion plot shows the four dif­fer­ent mod­els of Delta Loop with pat­terns ori­ent­ed broad­side (E/​W) at 30M above ground. The gain scales have been nor­mal­ized for direct comparisons:

  1. Black is a top-fed Delta Loop. Polar­iza­tion is horizontal.
  2. Blue is a bot­tom-fed Delta Loop, fed in cen­ter of bot­tom wire. Polar­iza­tion is horizontal.
  3. Green is a cor­ner-fed Delta Loop. Polar­iza­tion is vertical.
  4. Pink is a side-fed Delta Loop, fed 25% up the length of one side. Polar­iza­tion is vertical.

The mod­els show the hor­i­zon­tal­ly polar­ized anten­nas exhib­it 5 – 6dB over the ver­ti­cal­ly polar­ized anten­nas at 15 degree take-off angle. 6dB is a full S‑unit, and when sig­nals are weak this can mean the dif­fer­ence between noise or read­able S1 sig­nals. In this case, the bot­tom-fed loop is the supe­ri­or DX antenna.

By the way, there is lit­tle gain dif­fer­ence between top and bot­tom-fed Delta Loops. Save your­self the extra coax and feed from the bot­tom instead of top.

There is a deep null in the azimuthal pro­jec­tions to the N/​S with the hor­i­zon­tal­ly polar­ized loops, thus the side- or cor­ner-fed loops would be much bet­ter per­form­ers favor­ing these angles (see pre­vi­ous azimuth plots). This might mean the dif­fer­ence between the oth­er per­son hear­ing you or not at these com­pass points.

As to which is the best anten­na, I can argue the best-behaved anten­na will be the bot­tom-fed (hor­i­zon­tal­ly polar­ized) Delta Loop. From Roma­nia, this anten­na would favor Asia/​Oceania and Cen­tral/­South-Amer­i­ca. Of course, I am sure it would also do well into the East­ern Seaboard of the Unit­ed States. This anten­na will not at all favor Canada/​Western Unit­ed States nor Africa.

In sup­port for the ver­ti­cal­ly-polar­ized Delta, it would seem a bet­ter all-around per­former albeit an S‑Unit down E/​W from the bot­tom-fed loop. To the E/​W it will per­form with a few dB gain over a tra­di­tion­al ver­ti­cal, and will also pro­vide equiv­a­lent per­for­mance to a ver­ti­cal shoot­ing N/​S.

The abil­i­ty to switch feed point loca­tions is desir­able. Auto­mat­ic switch­ing of the feed point can be a bit of a design chal­lenge (one I have been con­sid­er­ing for about a year). If ready access to the base of the anten­na is con­ve­nient, a man­u­al com­pro­mise might be easy to imple­ment. If you were to feed the anten­na using coax­i­al cable, two coax­i­al feed point insu­la­tors might be installed in the bot­tom wire: One at the mid-point of the wire, and one at the cor­ner. One of the coax­i­al feed-point insu­la­tors would be short­ed using a jumper (or coax­i­al plug wired appro­pri­ate­ly) while oth­er the feed-point is fed.

To con­clude, the bet­ter anten­na at this height is the bot­tom-fed Delta Loop. Ver­ti­cal­ly polar­ized anten­nas at this height above ground exhib­it an “in-between” ver­ti­cal take-off angle, either favor­ing much high­er or low­er place­ment of the anten­na sys­tem. As a hor­i­zon­tal­ly anten­na is high­er above ground, well-formed lobes of radi­a­tion occurs. The well-formed lobes, how­ev­er, can be prob­lem­at­ic if one desires recep­tion in non-favored direc­tions: Well-formed lobes also mean well-formed nulls! In this case, the side- or cor­ner-fed Delta Loop is more ver­sa­tile at the expense of an S‑unit broad­side. Obvi­ous­ly a means to switch the feed point to switch polar­iza­tion pro­vides the best utility.

In my expe­ri­ence, a Delta Loop is a fan­tas­tic per­form­ing anten­na. I have a cou­ple of corol­lar­ies based upon my expe­ri­ence and sup­port­ed by modeling.

  1. If you have space for an Invert­ed V anten­na, a Delta Loop anten­na at the same height is pre­ferred (if there are no phys­i­cal obstacles)
  2. A side-fed Delta Loop is a clear­ly supe­ri­or to a tra­di­tion­al Ver­ti­cal antenna.

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