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Author Topic: Adding legs to a ladder-line fed doublet?  (Read 2595 times)

Posts: 11

« on: April 10, 2013, 11:28:43 AM »

I have a theory question for all you antenna gurus out there...  Say I have a 102' doublet fed with 450 ohm ladder line (not a G5RV but just a plain ole doublet)...  Probably an average of 25' above ground due to HOA restrictions... ( Embarrassed)  Using a tuner in the shack for matching.

I understand that as you move up to the higher bands, you lose much of the broadside radiation and start developing a lot of lobes and nulls and eventually it can become slightly directional off the ends...

Knowing that the antenna is being fed with ladder line, could you add additional legs for the higher bands (aka: a fan dipole) to help maintain the broadside directionality on the high bands?  (Doublet is oriented N-S so I'd be trying to maintain E-W directionality...)

Or is 102' short enough so it doesn't get too directional off the ends on 10m?

I haven't yet ventured into modeling with EZNEC yet so educated guesses are welcome!


Posts: 624

« Reply #1 on: April 10, 2013, 12:00:02 PM »

Hi Josh

Yes I had to readjust things to metrical sizes.
And the answer is that there will be more lobes on 10 and 12 meters.
There will be problems of direction.
But As far i can tell , these lobes and nulls won't be a to great problem.
If you run your antenna in NEC programs the nulls will be much deeper then they will be in real life.
This is due to influences of conducting materials in the neighborhood of your antenna.

And of course you can put more legs of wire on to your dipole.
There will be only one big trouble how will you know what dipole did you bring in resonance, after you tuned the antenna?

Would it not be smarter to put up another small dipole up fed with ladderline for bands like 10 t/m 15 or 17 mtrs.
That dipole only has to be 2 x 3.30 meters (2 x 11) foot or may be a bit longer.
Then you have 2 feedlines to tune the antennas separately.

I use my 2 x 7 mtr rotatable dipole for 10 t/m 40 mtrs and even get it to work on 80 mtrs but that will cost me about 10 dB due to ladderline losses and tuner losses.
So 2 x 11 Feet of copper wire should be enough for 10 t/m 20 mtrs.

I do have a wire dipole of 2 x 12 mtrs (37 feet) with lengthening spiderweb coils at each end of about 8 meters of wire (25 feet),
 I use for 80/40 mtrs.
Both antennas are fed with their own 35 ft of 600 ohm homebrew copperwire made 600 ohm ladderline

I just happen to have 2 balanced tuners so each antenna has its own tuner which are connected by a coaxial antenna switch before I hook them up to my rig.

My homebrew relay operated antenna switch even gives me the opportunity to choose on which antenna and tuner, I would like to TX and on which I would like to RX.
Switching is done automatically by the rig by TX ground.

73 Jos

Posts: 17476

« Reply #2 on: April 10, 2013, 12:29:53 PM »

On 10m your antenna will have lobes at about 40 degrees from the direction of the
wire, and smaller ones broadside to it.  (The pattern will be the same as the G5RV
since the radiator is the same, regardless of the feed method.)

You can add another set of legs parallel to the first set, but it will require some care
to get a consistent pattern across multiple bands.  Because the two antennas are
fed in parallel, the relative currents in the two wires will depend on the impedances,
which will vary from band to band.  You want most of your current to flow in the
shorter wire on the higher bands, so you need a wire that has a significantly lower
impedance than the other wire.  You might get by with something like 1/4 wave
wires for 12m (making a 12m dipole), which might work for both 10m and 15m, but
I'd have to model it to see the plot of impedance vs. frequency.

Depending on what combination of bands you want on the added wire, is isn't

Posts: 477

« Reply #3 on: April 11, 2013, 05:21:25 AM »

You are right, the higher you go in frequency the more lobes/nulls you can have.  And the radiation can change from broadside to end-fire.  But, at the height you have the antenna you are not going to have much 'directionality' at all, certainly not on the lower HF bands.  Above 20 meters maybe, but even still those 'lobes'/'nulls' aren't going to be very pronounced.  For them to become very directional at all most horizontal antennas have to be something like 1/2 to a full wave length above ground/dirt.  Your antenna will probably have a more omni-directional pattern than anything.
 - Paul

Posts: 4310


« Reply #4 on: April 11, 2013, 06:27:17 AM »

The 102' dipole is non-resonant on most HF bands which means that a resonant fan dipole element on those bands will accept most of the RF signal and radiate it mostly broadside. However, the 102' dipole is resonant at 1.5WL on 20m so a parallel 1/2WL dipole will only absorb a little over half of the RF energy. The result on 20m would probably be six medium energy lobes with nulls in between, i.e. maybe not a good idea for 20m unless that radiation pattern is desirable.

73, Cecil,

Posts: 2439

« Reply #5 on: April 11, 2013, 06:54:13 AM »

How directional the antenna is depends on the elevation angle.  If you only look at 90 degrees elevation, NO antenna is directional.  A 102 ft dipole at 25 ft height and at 15 degrees elevation should have a MAXIMUM ratio of lobes to nulls of about the following:

80 - 3.5 dB
75 - 4.5 dB
40 - 12.5 dB
20 - 18 dB
15 - 27 dB
10 - 37 dB

As PA1ZP said, other things effect the nulls.  Surrounding objects, drooping wires, antenna asymmetry, common mode currents. etc all tend to fill in the nulls.  If you account for those things in the model, then the model will show the results.  In the real world it is highly unlikely that you will ever see that 37 dB ratio on 10 meters or the 27 dB on 15 meters, but what you will see will be a large difference.

 Jerry, K4SAV

Posts: 11

« Reply #6 on: April 11, 2013, 06:53:51 PM »

Thanks for the input everyone!

It was something that was bouncing around my head and I was debating on trying it, but in the end it sounds like it won't really get me much more bang for the buck at that height...


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