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Author Topic: Center fed 1/4 wave 20 meter dipole project  (Read 7915 times)

Posts: 67

« on: October 23, 2013, 12:50:38 AM »

I really need help from the Elmers.

Project to build a 1/4 wave, 20 meter center fed dipole for camping.
$10 tripod, $10 16' aluminum telescoping mast.
Going to use aluminum tent poles ($28.00) @ 8.5 feet per leg.   I have yet to purchase any materials.

The wall I keep hitting is the feedline; it would seem that the impedance matching will be problematic with high losses.

Does anyone have any suggestions for this build?

p.s. please don't tell me to just make a 1/2 wave dipole....I wanna see it this can work.

Posts: 1845

« Reply #1 on: October 23, 2013, 01:17:44 AM »

Your quarter-wave dipole will have a low radiation resistance and a very high capacitive reactance, giving an SWR(50) of about 560:1, so yes your problem is how to match it and feed it efficiently. If you were to feed it directly with 20ft of RG58 coax you'd lose 96% of your power in the feedline!

If you really want to try a quarter-wave dipole, use open wire line to feed it - 20ft would have losses of only 17% and the impedance at the radio end should produce acceptable losses in a tuner.

Steve G3TXQ

Posts: 7718

« Reply #2 on: October 23, 2013, 01:25:46 AM »

Your dipole will be a 1/2 wavelength dipole but will be shortened to a length of 1/4 wavelength.

It will need to be inductively loaded to resonance (1/2 wavelength). The loading coils can be placed anywhere from the feedpoint to halfway along each dipole wire. Placed at the center the radiation resistance is 14 ohms and at the center 25 ohms. The coil inductance is 5.1 and 9.4 uH respectively.

There are several ways to transform the input impedance to 50 ohms. A Beta match (shunt inductor), 1/4 wavelength matching line, or Gamma match are three ways.

For the center loaded dipole a shunt matching inductor of about 0.5 uH is connected across the feedpoint. It is adjusted along with the loading coils (or the wire length beyond the coils) to obtain a 1:1 VSWR.

For the center loaded dipole the matching line can be a 1/4 wavelength of 37.5 ohm coax. This is made by paralleling two lengths of 75 ohm coax. Using foam dielectric coax having a velocity factor of 0.82 the physical length is 14 feet. The only adjustment is either the loading coil inductance or the wire length beyond the coils. For each inch trimmed off each wire the resonant frequency increases by 75 kHz.

For the dipole with loading near the feedpoint the matching line is 25 ohms and is made by paralleling two 50 ohm coaxial cables. The trimming sensitivity is also 75 kHz per inch. The matching line length is 14' for foam dielectric coax and 11.5' for solid dielectric coax.

« Last Edit: October 23, 2013, 02:19:20 AM by WX7G » Logged

Posts: 7

« Reply #3 on: October 23, 2013, 02:58:16 AM »

Can it work? Sure, probably, with a bunch of fiddling. The more appropriate question - which you don't want to hear - is "Why bother?"

I mean, I dig what you're trying to do. Portability is cool. But you can get some telescoping whips from MFJ, stick 'em in the end of your aluminum legs, and have a just-as-portable 1/2-wave rotatable dipole and be done with it.

73 de Bob WP2XX

Posts: 14495

« Reply #4 on: October 23, 2013, 05:07:19 AM »

If you use loading coils about 2/3 out on each leg then the dipole will be electrically resonant and you can feed it directly with 50 Ohm coax and a 1:1 balun. Take a look at the BudiPole design.

Bob  AA4PB
Garrisonville, VA

Posts: 67

« Reply #5 on: October 23, 2013, 06:11:22 AM »

Thank you very much for the info...just what I was looking for.

What a wonderful resource eham is!!

Thanks again for the information.

I think I'll try the MFJ whips.

Posts: 5483


« Reply #6 on: October 23, 2013, 06:27:04 AM »

My portable vertical is a 16' crappie pole with a wire that's divided into lengths for 10, 15, 17 and 20M and connected together with spade terminals.  No tripod, just ~4' strings for guys on the bottom-most section of the pole.  24 ~15' radials lay out in a spoke pattern from the base.  Fed with 8x coax, with a choke unun at the feedpoint.  Used it for JOTA this past weekend, worked like a hose.  Changing bands requires laying it down and connecting/disconnecting the terminals for the desired band but of course does not need a tuner.  Cost of all parts new <$20.   Packs into a small carboard box, plus the collapsed pole.

Mark K5LXP
Albuquerque, NM

Posts: 17483

« Reply #7 on: October 23, 2013, 10:10:19 AM »

Another method for matching the half-sized 20m dipole (or a full-sized 20m dipole
on 40m if you end up using the telescoping whips).  You can use this for just
about any shortened antenna, whether it uses wire or stiff elements.

Calculate the required loading coil for the center of the antenna.  The data for
base-loaded mobile antennas may give you a good starting point.

Wind the required coil on a length of PVC pipe, or a similar former.  If you can't
calculate the required value, make a guess, possibly based on a similar design
for a different band.

Insert the coil in series between the two halves of the element.

If you have a dip meter handy, couple it to the coil and find the resonant
frequency.  Adjust the coil and/or length of the elements until the antenna is
resonant close to the desired frequency.

Wind a few turns of insulated wire around the loading coil and connect it
across your coax.

Now by adjusting the loading coil to move the resonant frequency, and the
number of turns on the link coil to adjust the SWR at resonance, you should
be able to find a good match.

Splitting the loading coil and putting part of it in the middle of each side of
the antenna, rather than all in the center, generally increases the efficiency,
but this method is a bit easier and will handle a wide range of short antennas.

Posts: 624

« Reply #8 on: October 23, 2013, 02:13:40 PM »

Hi k8eyc

At the hight of only 16 feet it ould be much smarter and more efficient to build a simple vertical groundplane with a few wire elevated radials.

Place the GP at about 3 to 5 foot feedpoint hight and use simple copper wire for 3 radials.
I have build a lot of these GP's out of old aluminium half waves for 11 meters.
i build about 10 of these out of old 11 mtr halfwaves and about 5 new ones for 20 mtrs.

Cheap and easy can not go wrong with a simple GP.
The GP will radiate much lower and will work better on DX.
i build about 100 GP's from 70 cm up to 80 meters full size.
These are hounest and reliable antennas.

And you do not have the problem of matching and using coils to get the antenna up to size and SWR.

73 Jos
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