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Author Topic: Double Delta Antenna  (Read 18519 times)
N3DT
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« on: July 02, 2014, 03:25:23 PM »

I've build a double delta antenna, mounted vertically, with the upside down delta on top and the lower delta normal and attached at the same feedpoint.  My reasoning was if a full wave loop presents 120 ohms, adding another would half the Z to around 60 ohms, not to bad for either 75 ohm or 50 ohm coax.  It built it for the low end of the FM band for some stations I listen to there that are about 60 miles away.  I've been using a dipole with a home made pre-amp that's been satisfactory, plus it's got a splitter for 2 radios and if the DD worked any better I'd use the pre-amp.

So I mount the DD not far from the dipole and run 100' of RG6 to it and sweep the freq and it's resonant around 90MHz, but the matching Z is around 25 ohms and it's got a pronounced swr dip, that is it's not very wide BW.  Sweeping the dipole (3/4" AL) gives me a much wider BW, matches at 75 ohms, but I'm getting some strange resonances, it may be coupling to the DD since it's only a few feet away.

Thing is I'm wondering about the 25 ohms.  Both antennas are mounted near the side of the house broadside due east about 4' away from the aluminum covered sub siding.  I'm wondering if the DD is getting affected by the foil?  I'll try moving the antenna away from the house and dipole.

Both antennas work good enough for full quieting without the pre-amp, I have a digital RX that measures SSI and also carrier to noise ratio, both antennas are within 4-5dB C/N with the dipole inching out the DD at around 68-72 dB.  I was hoping the DD would be a bit more efficient since it's so much more wire?  It was easy enough to make with a couple pieces of pvc and a long piece of wood.

This is a prequel to making a DD for 6M and hauling it up about 50-70'.

Comments?

Dave
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G4AON
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« Reply #1 on: July 02, 2014, 03:51:43 PM »

You don't mention the spacing between the elements. I haven't experience of double delta antennas, but I do have some quad experience... Normally, a 2 element quad (and delta, I guess) would be expected to have a feed impedance in the region of 100 ~ 200 Ohms and can be configured to give 100 Ohms in order to easily match to 50 Ohms with a quarter wave of 75 Ohm coax. The usual method is to try different driven to reflector spacing and element sizes to obtain the wanted impedance... Fairly easy with software such as EZNec.

Some older designs used a gamma match to obtain a 50 Ohm feed, however a quarter wave of coax is a more elegant and robust matching arrangement, with probably less loss too. I use such an antenna for 6m with a near perfect match (http://www.qsl.net/g4aon/quad/).

I suspect your issue may be due to the close proximity of other antennas and/or sheet metal.

73 Dave
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KB4QAA
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« Reply #2 on: July 02, 2014, 04:15:32 PM »

I'm not clear about your configuration.

-Two delta loops fed in parallel?

-Bow Tie or Biconic (skeleton) with one side fed against the other?
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KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
N3DT
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Posts: 1746




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« Reply #3 on: July 02, 2014, 05:30:06 PM »

Like this.



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WB6BYU
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« Reply #4 on: July 02, 2014, 06:18:32 PM »

That's a good approach, but sometimes you may have to adjust the ratio of the flat end to the sides
of the delta loop to get a good match (especially if used with a reflector.)

Hmm...   I don't find a single-element version in my EZNEC files.  I'll see if I can come up with one
after dinner to try to reproduce your results.


[added after dinner]

Well, my 2m model seems to work just fine.  Equilateral sides of ~30" gave 82 ohms at 144 MHz. 
By making the ends narrower (26") and the sides longer (30.5") I got 54 ohms at 146.5 MHz.

So I have to ask, how did you measure the feedpoint impedance?  How long was the coax
between the feedpoint and the SWR analyzer?
« Last Edit: July 02, 2014, 06:53:29 PM by WB6BYU » Logged
N3DT
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Posts: 1746




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« Reply #5 on: July 02, 2014, 07:39:21 PM »

I measured the Z with a return loss bridge through 100' of RG6, so it's really hard to say what the Z at the feed point really is.  It did get a good SWR match with a 25 ohm reference with that setup. The dipole matched good with 50/75 ohms, same setup/RLB. If it stops raining tomorrow (unlikely) I can move the antenna to the porch where it's pretty much in the clear, away from the foil covered insulation and even get the SM next to the antenna and a short piece of RG6 from the antenna to the SM/RLB.

I've been switching it tonight on the FM stations with a coax relay and can hardly tell the difference with the dipole and DD.  I would have expected the DD to do better.

Are dipoles really that much wider in BW than full wave loops?
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K4SAV
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« Reply #6 on: July 02, 2014, 07:49:21 PM »

It's unclear as to whether you are measuring the antenna with an instrument calibrated for 50 ohms or 75 ohms.

When measuring thru a 100 ft length of RG-6 there will be many points where the reactance goes to zero.  These will be fairly close together.  You should be able to find one where the reactance goes to zero and the impedance is close to 25 ohms but it probably won't be the minimum SWR point.   (That will be difficult to determine if the instrument is calibrated for 50 ohms.)

The feedpoint impedance of the antenna should be close to 73 ohms.  When you plot a frequency sweep relative to 50 ohms thru 100 ft of RG-6 you get a whole bunch of ripples and these ripples appear to have narrow bandwidth.  It's difficult to tell exactly where the antenna is really resonant when looking at a 50 ohm plot. If you plot this relative to 75 ohms, all the ripples go away, the SWR curve becomes very smooth and the bandwidth appears to be very wide.

I finally solved this problem with my analyzer and made a 75 to 50 ohm adapter for it.  The SWR is then good but I have to remember to apply a multiplier to the resistive reading.

Jerry, K4SAV

Edit:  The SWR curve will be even worse if measured relative to 25 ohms.  You won't even be able to determine the resonant frequency using that reference.  Use a 75 ohm reference and see what you get.
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N3DT
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« Reply #7 on: July 03, 2014, 07:41:12 AM »

The RLB I'm using I can select what ever reference I want to use.  Here's the circuit.  I think I got it from a broken HP or Tecscan RLB many years ago.



So yes, it's going to be hard to say what the Z at the antenna feed point is through 100' of RG6.  I initially used a 50 ohm ref and the swr was above 2:1 at resonance, so I tried a 75 ohm and it went up, so I tried a 25 ohm and the match went down close to 1 at the dip.  But yes the RLB is made to use a 50 ohm input.

I'm using a Moto service monitor in the RF sweep mode which is adjustable from near 0 to 10 MHz width from 0 to 1GHz CF.  I feed the DC output of the RLB to the scope input on the SM and use resistors to calibrate swr points against the ref.  I learned this back in the 70's when I worked at an antenna company and they had lots of Tecscan and HP test equipment.

I wish I had that tunable Vector Voltmeter they had.  I think it would tune up through 30 MHz or so.

I'll take the antenna out to the porch where it's away from the house a bit and take the equipment right up to it and use a short piece of coax to test it and see what I come up with.
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WB6BYU
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« Reply #8 on: July 03, 2014, 09:04:20 AM »

Quote from: N3DT

So yes, it's going to be hard to say what the Z at the antenna feed point is through 100' of RG6.  I initially used a 50 ohm ref and the swr was above 2:1 at resonance, so I tried a 75 ohm and it went up, so I tried a 25 ohm and the match went down close to 1 at the dip.



That just tells you the impedance at the end of the transmission line where you are taking the
measurement.  To find the impedance at the antenna you have to correct the value for the length of the
transmission line.

Are you determining resonance by minimum SWR/return loss, or by finding where X = 0?  The latter
doesn't work through a length of transmission line (again, unless you correct your complex impedance
for the electrical length and losses of the line.)  Rather, the point of minimum return loss with the
bridge terminated in 75 ohms would be the best indicator.

A balun at the feedpoint is a good idea, too.

The double delta can give around 3dB gain over a dipole, but when hung vertically (for horizontal
polarization) the average height is less than a dipole for the same maximum height:  it should
provide some advantage when the bottom wire is up at least 1/2 wavelength.

It can make a useful directional antenna when backed by a similar reflector:  by making the elements
a bit longer and narrower you can get a good match to 50 ohms.
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N3DT
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« Reply #9 on: July 03, 2014, 10:36:44 AM »

I'm not sure what's going on. I've moved the loop over to the deck in the open.  I made the loops each 11'8" total (1032/88.5), 3'11" each side.  It's acting like all I'm looking at is the peaks and valleys while sweeping the frequencies on an open piece of coax. I did put a resistor at the antenna end of the cable and I measure that plus the length of wire on the ohmmeter, so it's not open. There really doesn't seem to be any resonance.  Comparing the dipole and the double loop on the spectrum analyzer the dipole stations seems to be about 20 dB stronger than the loop.  It just doesn't make any sense.  I wonder if the freq is beyond the capacity of my home made RLB?  The dipole even looks strange on it but it does seem to have a resonant point. Very different curves when comparing it to 75 vs 50 ohms though.  It may be acting funny because it's so close to foil sub siding.

I have used this RLB on little ground planes at 2M and 70cm and it seemed to work there.  Here's what they look like, about the same size as the HP or Tecscan ones and like I said I opened up a bad one and copied it.



Maybe I'll make another simple wire dipole and try that to start. I could also try disconnecting the lower loop and see what happens.  Yeah I don't have a current choke for that freq I'm just clipping on to the feed point with one of those BNC to clip lead things.
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N3DT
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« Reply #10 on: July 03, 2014, 12:46:33 PM »

I still don't know what's going on.  I tested a 2M ground plane with 12' of RG55 and it's fine.  It does look strange when I ref it to 75 ohms, but that's to be expected.  I put the GP at the end of the 100' of RG6 and it gets real strange, especially with a 50 ohm ref. It looks like I'm again sweeping an open piece of coax, with a resonance around 2M.  If I use 75 ohm ref, the peaks and valleys flatten out and the 2M GP of course looks real strange.  I think I'm messing up mixing 50 and 75 ohm stuff.  But it still doesn't explain the loop.  I disconnected the lower loop and it still looks like I'm just sweeping an open piece of coax no matter which ref I use.

I'm going to punt for now and think on it.
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N3DT
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« Reply #11 on: July 03, 2014, 01:18:53 PM »

Duh, double Duh.  I've figured out most of the problem.  I've put a bolt through a piece of pvc to attach the feed point and forgot that the bolts shorts everything out, so I've been looking at a piece of coax with a short at the end, not much different than looking at a cable with an open end.  I hate to admit this.  Anyhow, I've got something to work at now, I just need to figure out a simple way to make the feed point that doesn't involve shorting it out.  It's got to stop raining though.



Dave
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K4SAV
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« Reply #12 on: July 03, 2014, 02:42:59 PM »

Duh, double Duh.  I've figured out most of the problem.  I've put a bolt through a piece of pvc to attach the feed point and forgot that the bolts shorts everything out, so I've been looking at a piece of coax with a short at the end, not much different than looking at a cable with an open end.  I hate to admit this. 

Not unlike the guys that attach the radials to the bottom of the vertical.   Smiley

When measuring thru a long length of coax, you are going to get a smooth curve only when the antenna is a close match for the coax and you are measuring the SWR relative to the coax impedance.  Other combinations will give curves that you may not be able to figure out.  This antenna should be close to 73 ohms so measuring relative to 75 ohms should give something understandable.

Look at the plot below.  All these curves are for this same antenna with 100 ft of RG-6 but measured relative to 25, 50, and 75 ohms.  Big difference in the plots.

Jerry, K4SAV

[/URL]
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N3DT
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« Reply #13 on: July 03, 2014, 04:01:22 PM »

We'll see what happens when I get the feed point straightened out.  I'm sure it will be lots different.  I'm surprised I get only a 20 dB difference between a shorted coax and the dipole.  That should have been my clue right there.
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WB6BYU
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« Reply #14 on: July 03, 2014, 05:07:56 PM »

Quote from: N3DT

...I just need to figure out a simple way to make the feed point that doesn't involve shorting it out...



One way is to put the holes in the PVC near one end, then stick a screw through it from the inside.
Sometimes I've managed to do that while connecting the coax on the inside (using lugs crimped
onto the cable ends).  You can do something similar with a pipe cap, with the two bolts sticking
out the top or sides.

I find it is easier to use a scrap of PVC garden lattice, since it is flat and mills easily into whatever
sort of antenna insulator I want.  But then I have a good supply of pieces on hand.
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