Delta options

[KD6KWZ]

OK, I found a long fiberglass pole, so I tried a stealthy Delta loop on 10 meters in the patio area.  Inspired by this:

http://www.w5sdc.net/delta_loop_for_hf.htm

36 feet of around 30 gauge copper wire, yes, enamel removed off the connected end. The raw (tuner bypassed) SWR was just awful, like 8:1. I tried hooking one end of the delta to the second feedline terminal of the manual ET-1 tuner, and also tried leaving it just open, with using the "hot" to the wire output of the tuner. Yes, I was able to tune it, but the tuner became VERY frequency sensitive, readjustment needed for more than a few KHz of frequency change. I had 3 turns of the feedline coax between the rig & the tuner in a 6 inch diameter loop. Yet, I still had a report of RF on my transmitted audio. I am operating very close to the antenna. Note this is only 22 watts PEP on SSB.

I am using a 2 foot rod in moist earth for the ground.

So, I need some ideas:

1. Am I loosing a lot of RF in the tuner if the "raw" SWR is so high?
 
2. Do I need a balun? Is the base Delta loop too high in impedance to match to the 50 ohm feedline, or the tuner, without one??

3. Would a 1/4 wavelength of 75 coax do better than a balun in matching the loop to the feed line?

4. Do I need more turns of feedline choke than the 3 turns of 6 inches of diameter to reduce RF? That article says 6 to 8 turns. Yes, I know need to get farther from the antenna, in any case.

5. Would a clip on ferrite choke on the feedline work better than those 6-8 turns of feedline loop to keep RF out of the audio?

[K5LXP]

I tried a stealthy Delta loop on 10 meters in the patio area.  Inspired by this:
http://www.w5sdc.net/delta_loop_for_hf.htm

The configuration in the topmost illustration? Or one of the ones illustrated just below that?

The raw (tuner bypassed) SWR was just awful, like 8:1.

It shouldn't be.  That would lead me to suspect you are having effects from objects in the proximity of the antenna, you took a snapshot of SWR far off-resonance, you're too close to the ground or there's a measurement error.  Have you swept the band to see if there's a dip in resonance anywhere?

the tuner became VERY frequency sensitive, readjustment needed for more than a few KHz of frequency change.

That could be due to tuning an antenna at the limit of the tuner, or possibly common mode.  You need to solve the issue with the SWR being high first before you can deal with tuner issues.

I had 3 turns of the feedline coax between the rig & the tuner in a 6 inch diameter loop. Yet, I still had a report of RF on my transmitted audio.  I am operating very close to the antenna.

If you're operating in the EM field of an antenna, it won't matter what kind of balun you're using if your transceiver is sensitive to that.  Your RFI could be coming directly off the antenna.

  I am using a 2 foot rod in moist earth for the ground.

I would eliminate this.  It's not a safety ground, RF ground or effective surge ground.  Resolve your antenna issue before you add more variables.

1. Am I loosing a lot of RF in the tuner if the "raw" SWR is so high?

You have an antenna problem.  You need to resolve that before you worry about tuner losses.

2. Do I need a balun?

Technically, yes.

  Is the base Delta loop too high in impedance to match to the 50 ohm feedline, or the tuner, without one??

Nominally your feed Z should be around 100 ohms.  Not too high in my opinion.

3. Would a 1/4 wavelength of 75 coax do better than a balun in matching the loop to the feed line?

It would obviate the use of a separate tuner.  It wouldn't do it "better".  And your loop has to be working right in the first place.

4. Do I need more turns of feedline choke than the 3 turns of 6 inches of diameter to reduce RF? That article says 6 to 8 turns. Yes, I know need to get farther from the antenna, in any case.

Adding 3 more turns isn't going to resolve your problem.  Divide and conquer - fix your loop, decide how you'll tune it, then determine if coax balun can offer enough isolation for your installation.  It may not be solvable if your radio is susceptible to being in the EM field.

5. Would a clip on ferrite choke on the feedline work better than those 6-8 turns of feedline loop to keep RF out of the audio?

You might try one on the mic and power cables first.


Mark K5LXP
Albuquerque, NM

[WB6BYU]

...The raw (tuner bypassed) SWR was just awful, like 8:1. I tried hooking one end of the delta to the second feedline terminal of the manual ET-1 tuner, and also tried leaving it just open, with using the "hot" to the wire output of the tuner.

I'm very confused about how you are feeding this - are you running coax up the pole
(as shown in the illustration on the link) or connecting it directly to a tuner?  How do
you have a free end to connect to your tuner?  How do you decide which side of the
loop is "hot"?

You should have one end of the loop wire connected to the coax center and the other end
to the coax shield at the feedpoint.  Yes, you can use a balun, but you should be able to
get a reasonable SWR (under 3 : 1) without one.  At that level, the losses in your coax
and in your tuner will be low.

Or, if you are sitting right at the feedpoint (which is more likely if you aren't feeding it
at the top of the pole) then you can connect the two wires from the loop to the
balanced output of your tuner.

I just modeled an antenna with the dimensions you described, and without any matching
the 5 : 1 SWR bandwidth is 27.7 to 29.7 MHz, with a minimum of 2.3 : 1 at 28.7 MHz.
If your dimensions are off, or there are other objects near the antenna, the SWR could
be higher.

It sounds to me as though you don't have it connected properly.

1. Am I loosing a lot of RF in the tuner if the "raw" SWR is so high?
 

Once you get the antenna working properly, losses in the coax and in the tuner will be low.

2. Do I need a balun? Is the base Delta loop too high in impedance to match to the 50 ohm feedline, or the tuner, without one??

You don't need a balun, but it will help to reduce common mode currents (flowing on the
outside of your coax.)

A 4 : 1 balun at the feedpoint should give you an SWR less than 2 : 1 at resonance.  Or it should
be fine with a tuner in the shack instead - most should have sufficient matching range.

3. Would a 1/4 wavelength of 75 coax do better than a balun in matching the loop to the feed line?

The purpose of the balun is to reduce currents on the feedline.  If that is a problem, you
want to use a balun whether or not it is part of your matching system.

The quarter wave of 75 ohm coax makes a convenient match - especially if that is all the
coax you need to feed it.  The length isn't particularly critical: a +/- 1 foot is close enough.
And you can make it odd multiples of a quarter wave - for example, a 3/4 wave line is about
21' (for foam coax) but changing the length from 19' to 23' is close enough that the normal
antenna tuning will take care of it - it just shifts the resonant frequency slightly.  (But 21'
gives an almost perfect match at resonance with foam coax - it will be shorter with
solid dielectric lines such as RG-59.)

Actually, you can feed the antenna in the middle of the bottom wire just as the article
shows doing it at the top - that may be more convenient as your support post doesn't
have to carry the weight of the feedline and balun at the very top.  Contrary to what
the article claims for configuration (B), there is no difference in the radiation patterns. 
In that case, 21' of coax may be enough to reach the operating location.  Or just run
75 ohm coax all the way to the shack if you have some handy.  The losses will be lower
than using 50 ohm coax, though I wouldn't worry much about losses for a monoband antenna.

4. Do I need more turns of feedline choke than the 3 turns of 6 inches of diameter to reduce RF? That article says 6 to 8 turns. Yes, I know need to get farther from the antenna, in any case.

Let's get the connection problem figured out, then see if you still need a balun.

5. Would a clip on ferrite choke on the feedline work better than those 6-8 turns of feedline loop to keep RF out of the audio?

It depends on the number, type and size, and whether you can pass multiple turns of coax
through each core.


The delta with the point up is the easiest one to string up with a single support, putting
the point down as shown in configuration [C] improves the low angle radiation because
the average height of the antenna is higher.

[KD6KWZ]

are you running coax up the pole (as shown in the illustration on the link) or connecting it directly to a tuner?

I tried connecting it to the tuner's balanced line terminals. No coax to the Delta was tried yet. Maybe I'll do that next. I did try having one point being feed to the long wire terminal on the tuner, and ground the other point, but that didn't help.

If your dimensions are off, or there are other objects near the antenna, the SWR could be higher.

That's part of the problem. It's only about 12 feet away from my 2 story townhome. I can't get it further away. One side of the delta is supported from a wooden shed roof. I guess I need to find another fiberglass pole for that side.

Right now, it looks kind of like configuration (D) on that page, Apex up, corner feed. The sides are not perfectly even in length.

So, off to find another insulated pole & solder on some of the RG-59 to feed it...

Thanks for some more ideas.

[WB6BYU]

I tried connecting it to the tuner's balanced line terminals.

Could you lower the SWR by adjusting the tuner?

Was the high SWR measured through the tuner, or some other way?

A loop should be within the matching range of virtually any tuner with balanced output.
(Unless, of course, it requires a jumper between the "single wire" terminal and one of
the "balanced" terminals, and that wire is missing.  A common problem, actually.)

So IF you fed it straight from the tuner, and if the best you could tune it to was 8 : 1,
then there is a problem somewhere that has nothing to do with the antenna itself.

I don't have the manual for that tuner, but based on others of that vintage I'd make a
couple of observations:

First, to use the balanced connection you need a jumper wire from one side of it to the
single wire output.

Second, there is no "straight through" switch selection for the single wire output, which is
used for the balanced line.  So you can't measure the SWR without going through the
tuner.

[Here is my trick for that when I was feeding a loop that came right to my operating
table:  instead of running the jumper to the single wire terminal, I connected it from
one side of the balanced output to a banana plug that stuck into one of the coax
outputs.  That way I could choose to run that coax output straight through and feed
the loop through the balun.]

Connecting the loop from the single wire output to the ground connector should also
work - not balanced, but you should still hear signals.  But you still have no straight-
through option based on the front panel switch.

So unless you run a jumper to one of the coax output jacks, you'll have to adjust the
tuner to use the loop instead of running it straight through.  Otherwise it isn't surprising
that you can't hear any signals.  One option would be to take any piece of coax cable
you have handy with a PL-259 on one end, twist the ends of the wires onto the coax,
and plug it into one of the coax ports that allows you to run straight through.  (I always
have a couple pieces of coax around for doing this when I'm experimenting with antennas.)


At this point, don't worry about changing the loop itself.  Concentrate instead on
getting it connected properly to the tuner.  The tuner should be able to match the loop
easily whether you use the balanced output or the single wire to ground.

Of course, it doesn't hurt to check the loop for continuity with an ohmmeter to make
sure there isn't a break in the wire somewhere.

Good luck!

[KD6KWZ]

Was the high SWR measured through the tuner, or some other way?

There's a bypass mode on the tuner. In bypass, the SWR was 8:1.

I could tune it lower in the normal tuner mode, but the bandwidth was very narrow, a slight frequency change really shifted the SWR. And, I wonder how much power I was losing in the tuner if the base SWR was so high. I had the same base & tuned SWR results by hooking it to the balanced line terminals & using the wire terminal with only one side of the loop hooked up.

I could hear other stations, I even worked a JA & AB0. But, the signal reports were not so good.

Of course, it doesn't hurt to check the loop for continuity with an ohmmeter to make
sure there isn't a break in the wire somewhere.

It's enamel insulated wire, and it looks OK. I have not ohmed it yet.

[WB6BYU]

There's a bypass mode on the tuner. In bypass, the SWR was 8:1.

I don't have the manual for that particular tuner, but I suspect that the "bypass" mode
routes the RF to a port on the back labeled "bypass", which typically would be for a
dummy load or an antenna that didn't require the tuner.

The two ports marked "coax1" and "coax2" can be selected on the antenna switch to
go either direct or through the tuner.

The "single wire" port only appears on the right side of the switch, so it is only accessible
through the tuner.  To use the balanced wire terminals you need to add the jumper wire
between the single wire output and one of the balanced wire terminals.

At least this is what I expect, based on looking at a photo of the front of the tuner.


So if you were using "bypass" mode with nothing connected to the bypass port, and
could reduce the SWR with the tuner, you just good a good lesson that simply being able
to match something with a tuner doesn't guarantee that you have an antenna connected
at all.  And also something about propagation when 10m is open - it doesn't take much
power or a big antenna to make contacts.

[KD6KWZ]

The two ports marked "coax1" and "coax2" can be selected on the antenna switch to
go either direct or through the tuner.

You're right, what I said was bypass was actually direct mode, that is, no tuning, but SWR & power are still read.
There is a bypass jack, likely for a dummy load.

I purchased another fiberglass pole. I got the other point higher off the ground, attached some RG-58/U coax to
the loop, so it now looks like Configuration (C). In direct mode, the SWR is now 4:1. In tuned mode, I can get it
down to 1.1:1, and it's not crazy sensitive to slight frequency changes anymore. The tuner does have some trouble
above 29.3 MHz, the SWR only can be tuned to 2.0:1, so maybe the loop is a tad too long?

So, I'm getting closer. I also found the 12V power supply was impressing some hum when it was close to the
HTX-10, so I keep it far away as possible from the antenna, tuner, & rig.

Would a balun help anymore at this point?

[WB6BYU]

How does the SWR vary across the band?  The loop might be either too long or too short,
and knowing where the minimum SWR is would help (or at least whether the SWR is lower
at the upper or lower end of the band.)

With either a quarter wave of 75 ohm coax, or a 4 : 1 balun, you probably can get the
SWR down to where you don't need to use the tuner.

Would a balun help?  I haven't had as much common mode current with loops as I have
with dipoles.  Since this is a monoband antenna, it probably makes sense to use a coax
choke if you don't use a 4 : 1 balun for matching.  (See G3TXQ's website for suggestions
on the number of turns and the coil diameter.)  But in the meantime, go ahead and use
the antenna to see how it works.

[KD6KWZ]

OK, I shortened the RG-59 cable to 1.73 meters, that's the Velocity factor X 1/4 X wavelength at 28.5 MHz.
I installed a ferrite at the end of the RG-59 before the loop. I got this done just as the band was fading for
the day, but direct mode (no tuner, just SWR) came through at 1.2:1 at 28.5 MHz, not bad! I can tune it to
1.1:1 now.

In direct mode to the loop now:

Frequency MHz      SWR
28.0                    1.6:1
28.2                    1.45
28.35                  1.35:1
28.5                    1.2:1
29.0                    1.1:1
29.1                    1.15:1
29.4                    1.4
29.699                 1.6:1




Thanks for all the input. It's on 2 fiberglass tent poles, so I can put it up & take it down quickly. I'm using RG-8X
cable between the RG-59 match section & the tuner.

Now, to find a mountain top for portable operation. That might help DX a little, you think?

73,

Mike H.

[WB6BYU]

... direct mode (no tuner, just SWR) came through at 1.2:1 at 28.5 MHz, not bad! I can tune it to
1.1:1 now.

I wouldn't bother using the tuner at that point.  It isn't going to make any significant difference,
except the reflected power meter won't bounce around as much as you talk.

If you aren't going to be operating on 10m FM you could lengthen it a bit and improve the
SWR at the CW end (and improve the match in the 28.3 - 28.6 MHz range as well). 
Or vice versa.  But it looks just fine as is.

Now, to find a mountain top for portable operation. That might help DX a little, you think?

If the mountain has a rounded top, setting it up on one edge (or down the slope from
the top) may work better than right at the top itself.  This is due to how the sloping
ground changes the angle of radiation for a given height above ground.

For portable use you may find that stringing the loop up as a diamond on a single mast
with the sides of the wires tied out with strings is more convenient to set up in the field.