Folded Dipole and 300 Ohm Twinlead?

(1/1)

Bill Silverman:
I'm planning on throwing up my first antenna in many years and have been reading about the advantages of using twinlead.  I also read in the ARRL license manual about folded dipoles and how their impedence is inherently 300 ohm (300 ohm feedline to 300 ohm load: sounds good to me).

Some online discussions have pointed out several disadvantages of folded dipoles over traditional dipoles, mostly about lower gain of a folded dipole vs the standard type.  

My plans were to erect a 40 meter inverted V folded dipole above my roof, feed it with 300 Ohm Radio Shack twinlead, match it to Roger Tango's homebrew 4:1 balun before bringing into the shack with coax, and finally hooking it up to my Drake MN-4 tuner.

Is this overkill?  Anyone have personal experience using folded dipoles? Comments?

Dan:
Quote from: Bill Silverman on February 24, 2010, 09:02:26 AM

I'm planning on throwing up my first antenna in many years and have been reading about the advantages of using twinlead.  I also read in the ARRL license manual about folded dipoles and how their impedence is inherently 300 ohm (300 ohm feedline to 300 ohm load: sounds good to me).


Twinlead tends to be better for multiband feed of a regular doublet, but I'm not so sure for a folded dipole.

The nearly 300 ohm impedance of the folded dipole only happens on the fundamental and odd harmonics... that's fine, just like the fact that you can feed a dipole easily with coax on the fundamental and odd harmonics, like 40m and its 3rd harmonic 15.

But on the even harmonics of the folded dipole it presents a very LOW impedance instead of a very HIGH impedance like a regular doublet.  This increases the mismatch to the 300 ohm line and increases the loss substantially.

I checked with a 40m dipole fed on 40, 30 and 20 with 30 feet of 300 ohm line.  For a normal dipole the losses are under 1dB on each band.  For the folded dipole, losses are low on 40 and 30m, but nearly 3.5dB higher on 20m.  This is not as bad as feeding the folded dipole with coax on 20 (about 6dB extra loss), but it's still substantial compared to what you could have with a non-folded dipole. 

So while a folded dipole makes a convenient antenna if you want to run a matched 300 ohm line, it doesn't work as well multibanded, at least in some circumstances.

73
Dan

Bill Silverman:
Thank you, Dan!

I'll go ahead with the standard "vanilla" 40 Meter dipole and the 300 ohm twinlead. It should also be a little more "stealthy" than the folded dipole (a plus when trying to appease the XYL)

Bill in Pasadena

Dale Hunt:
Quote from: KJ6CVO

Some online discussions have pointed out several disadvantages of folded dipoles over traditional dipoles, mostly about lower gain of a folded dipole vs the standard type.


A folded dipole has essentially the same gain as a standard dipole on the
fundamental frequency.  Your 40m folded dipole fed with twinlead won't
work any differently on 40m than a normal dipole would, even one fed with
coax, except that you would need to use a tuner with it.

For just use on 40m a standard wire dipole in a reasonable inverted vee
will be a good match to 50 ohms, and with RG-213 coax the feedline loss is
about 1/4 of a dB - not enough to worry about.  A folded dipole fed with
300 ohm line might cut the feedline loss in half, but the total difference
is well below the threshold of detection.  Certainly with the possibility of
additional losses in the twinlead antenna, balun, tuner, etc. the folded
dipole actually could be down a bit compared to coax feed, but in practice
I doubt you would notice any difference between them.

A folded dipole steps the antenna impedance up by a factor of 4:  in free
space where an infinitely thin dipole is around 72 ohms, this would give
288 ohms at the feedpoint.  As an inverted vee closer to the ground the
impedance of a plain wire dipole will be about 50 ohms, so the folded dipole
would be closer to 200 ohms.  Depending on the length of the twinlead the
impedance at the balun likely would be between 200 and 450 ohms.

Even with the smaller RG-58 coax to an ordinary single-wire dipole you
wouldn't gain anything significant by using 300 ohm line on 40m.  The
twinlead can be especially lossy if it gets wet, though I've heard of
folks using car wax on it to keep the insulation from absorbing as much
water.

But while there is no benefit to using twinlead on 40m, it would make the
antenna much more efficient on 20m than using coax.  In addition, the
folded dipole has very poor current distribution on the even harmonics of
the fundamental - in this case 20m and 10m.

So for just 40m, direct coax feed would work as well (or better, given the
convenience of not needing an antenna tuner.)  For multiband use, twinlead
is a better choice, but you may find that a 1 : 1 balun works better (depending
on the length of the twinlead).  In either case, a single wire dipole will work
at least as well as a folded dipole in your application, and considerably better
for multi-band operation.  (Actually the losses on 40m can be higher using twinlead
than using coax, depending on the length.)

Navigation

[0] Message Index