On Fri, 25 Jun 2021 11:10:38 -0400, in <
MPG.3b3fbbe7f392a61a98982a@news.eternal-september.org>, Ralph Mowery <
rmowery42@charter.net> wrote:
I was looking at the new QST and noticed MFJ has an ad for an end fed
antenna that claims no tuner is needed to operate over many bands.
How well would that antenna work compaired to say an OCF or even a
sloping dipole mounted at the same position ? Main object is a simple
field day antenna that works.
Too late for Field Day this year, but End Fed Half Wave (EFHW)
antennas work fine... assuming they incorporate a 1:49 unun at the
feed point and assuming you use a tuner if necessary, and assuming
power is limited to an appropriate level commensurate with the rating
of the unun. The tuner in the typical rig is perfectly capable of
correcting any mismatch, which isn't severe, but might cause some rigs
to fold back power. The club I belong to put up two of them for Field
Day. We worked 40M/75M on one and 20M/15M on the other. It was also
supposed to work on 10M, but we never tried 10M.
These things work because the non-WRC bands are harmonically related.
They're perfectly harmonically related at the lowest frequency in each
band, but if you cut the EFHW to a frequency that isn't the bottom
edge of the lowest band it's designed for - such as the SSB portion of
the band vs CW - multiples of that frequency can fall outside the
higher bands.
For use with an amplifier the ferrite core in the unun can be
destroyed due to heat and over saturation in very short order if it
isn't sized appropriately. MFJ sizes based on ICAS, which is fine for
control of heating, but to prevent saturation the core needs to be
designed for the instantaneous peak (PEP) power. Not saying the MFJ
isn't sized that way, just saying that if someone designs an EFHW for
a lower cost than buying one ready to go, that's a consideration.
The WARC bands aren't harmonically related to the pre-WARC bands, so
this antenna is almost surely a very poor choice for the WARC bands.
What is the make up of the MFJ antenna ? They say it is about 130 feet
long and end fed and do not really tell if you need a counterpoise or
not.
You need a counterpoise... unless you like the idea of using the
shield of the coax as a counterpoise.
Our club found that the G5RV is not that good of an antenna compaired to
many others.
IMHO, a G5RV is OK on 20M, but generally requires a tuner elsewhere...
and might even require a tuner on 20M depending how far it strays from
the original G5RV design. The tuner in a typical rig probably isn't
adequate for other than 20M.
More than you may want to hear about the G5RV antenna follows.
There are G5RVs and then there are G5RVs. The original was designed as
a 3/2 wavelength dipole on 20 meters at a center frequency of 14.150
MHz. It was 102 feet end to end across the top connected to 34 feet of
525-ohm open-wire matching section, the rest of the feed line having a
90 ohm impedance. (Per the original design that isn't well followed
these days.) It should be installed 1/2 wavelength in the air - which
is basically high enough to get the base of the 34-foot matching
section off the ground. The feed line connected to the end of the
matching section really *should* be 90-ohm coax or 90-ohm open wire
line for best SWR at 14.150 MHz, but practical available values for
feeder are 50 and 75 ohms, and most will use 50 ohms. Expect a best
SWR without a tuner (on 20M) of about 1.8:1 in this case. G5RV himself
used 75 ohm feeder. On all bands other than 20M, expect to use a tuner
to achieve an acceptable match. It's a myth bordering on an outright
lie that the G5RV antenna will give low SWR on all bands without a
tuner. Without a tuner you can expect to see SWR of 25:1 at some
frequencies.
I've seen very few "G5RV" antennas that come close to matching the
original design. Most I've seen have used homemade 450 or 600 ohm
ladder line in place of the original 525 ohm open wire line... with no
change in the length of the open wire matching section. This is not
the G5RV that Louis Varney designed and it won't act like the
original... which didn't have low SWR across most bands to begin with.
It was just TUNABLE across most bands. See the article mentioned
below.
About baluns. Don't! They're a basic no-no in antenna systems where a
high reactance is present - and generally speaking on bands other than
20M the G5RV can be a rather reactive antenna. See the article at the
URL below for details. Baluns are a general no-no when it comes to
fixing problems with highly reactive antenna systems because at higher
SWR, like anything over about 2:1, their core losses increase
dramatically, their efficiency decreases dramatically, and they can
overheat to the point of destruction if enough power is applied. Balun
failure can have a devastating effect on your transmitter or power
amplifier. No balun on a G5RV - NEVER!
An analysis of the G5RV antenna by the inventor, Louis Varney, G5RV,
is at the following URL.
http://www.qsl.net/aa3px/g5rv.htm . You need to read this and
understand it and then weigh all advice you've received in light of
it. I think your first move is to make sure you have a real G5RV
because if you don't it probably isn't realistic to expect it to act
like one. You also may need to resolve the difference between the
original 20M design center frequency of 14.150 MHz and a desired one
of (let's say) 14.285 MHz if you want optimum performance on 20M. I
imagine (guessing) this will increase the mismatch on the lower bands,
and juggle them around on all bands - except 20M - but the antenna was
meant to be used with a tuner anyhow.
You can use 75 ohm CATV cable between the tuner and the base of the
open wire feeder to reduce losses in the coaxial feeder due to high
SWR, but I wouldn't try to push a kilowatt thru it. RG-11 with a solid
center conductor would be preferable to the stranded center conductor
variety. Remember, the tuner provides a conjugate match to the
transmitter, but it doesn't lower the SWR on the antenna side of the
tuner so losses due to high SWR remain. All other things being
optimum, with 75 ohm coax you could expect to see fairly close to 1:1
SWR on a portion of 20M without a tuner, whereas with 50-ohm coax you
can expect maybe 1.8:1 at best without a tuner. The article explains.
Always remember - on all bands except 20M the G5RV is a rather high
SWR antenna and no matter how well your tuner provides a match, a low
loss feeder should be used to reduce losses in the feed line when the
mismatch is large.
The part of the article about any impedance being acceptable for the
open wire matching section is misleading if a low SWR on 20M *without*
using a tuner is important to you. If you plan to always use a tuner,
and accept any increased loss in the feed line, only then does the
impedance of the matching section become less important because a
tuner will match it to the rig, tho it won't eliminate feed line loss
due to high SWR. Hams should know this, but I think the article could
have emphasized this point since the myths about the G5RV as an all
band, no tune, antenna are so pervasive.
73 de Jim, KB3PU
--
Jim H
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