I plan on building a short dipole (inverted V) for traveling in my tiny
RV. It will only be designed for 75M. The mast is 20' tall, and I wish
the legs to be about 24 feet each.

I still haven't decided where to place the inductors, but in any case, I
have a question about their design. This antenna will only be used at the
5 watt level with a FT-817 Yaesu; an LDG Z-11 tuner is also available, but
probably not needed.

If I make my 75M dipole legs for about 24 feet long, then I figure I
will need about 70 uH inductors if they are center located.

My Question: What would be the requirement for a 70 uH inductor that
only needs to work with 5 watts? Do I need open air inductor with spacing
between windings, or would a smaller multi wound coil with insulated wire
suffice?

Thanks for any opinions on this.


Ed K7AAT

Ed wrote:
I plan on building a short dipole (inverted V) for traveling in my tiny
RV. It will only be designed for 75M. The mast is 20' tall, and I wish
the legs to be about 24 feet each.

I still haven't decided where to place the inductors, but in any case, I
have a question about their design. This antenna will only be used at the
5 watt level with a FT-817 Yaesu; an LDG Z-11 tuner is also available, but
probably not needed.

If I make my 75M dipole legs for about 24 feet long, then I figure I
will need about 70 uH inductors if they are center located.

My Question: What would be the requirement for a 70 uH inductor that
only needs to work with 5 watts? Do I need open air inductor with spacing
between windings, or would a smaller multi wound coil with insulated wire
suffice?

Thanks for any opinions on this.


Ed K7AAT

Just about any inductor will withstand the power level, so your
tradeoffs are among size, weight, expense, weather resistance, and loss.
The higher the inductor Q, the lower the loss. But to get the maximum Q
you need a large, air wound inductor that stays dry -- any water between
turns of any kind of inductor will spoil the Q. I've made an antenna
similar to the one you describe (but half the size, for 40 meters) with
reasonably low loss using two large powdered iron cores for loading at
the feedpoint. It was conveniently matched by link coupling to the cores
which of course you won't be able to do if the loads are away from the
feedpoint. If you can tolerate the size and/or weight, the use of
multiple conductors -- fanned or parallel --, or (not as good) a single
large diameter conductor will reduce the loss two ways. First, it
reduces the conductor's I^2*R loss which can become noticeable in a
short antenna. Second, it reduces the amount of inductance you need and
therefore for a given Q reduces the amount of inductor loss. I think
you'll have trouble getting reasonable Q and therefore reasonably low
loss from a small, multilayer air wound coil. You're best off with a
single layer air core solenoid or, not quite as good but decent if done
with care, a single layer winding on a large type 2 or 6 powdered iron core.

Just remember to protect the inductors to keep water from between the
turns. Also, remember that just beyond the inductors the impedance is
pretty high, so keep the part of the antenna beyond the inductors, in
particular, away from leaves, branches, and the like, especially if
they're wet.

Just about any inductor will withstand the power level, so your
tradeoffs are among size, weight, expense, weather resistance, and
loss. The higher the inductor Q, the lower the loss. But to get the
maximum Q you need a large, air wound inductor that stays dry -- any
water between turns of any kind of inductor will spoil the Q. I've
made an antenna similar to the one you describe (but half the size,
for 40 meters) with reasonably low loss using two large powdered iron
cores for loading at the feedpoint. It was conveniently matched by
link coupling to the cores which of course you won't be able to do if
the loads are away from the feedpoint. If you can tolerate the size
and/or weight, the use of multiple conductors -- fanned or parallel
--, or (not as good) a single large diameter conductor will reduce the
loss two ways. First, it reduces the conductor's I^2*R loss which can
become noticeable in a short antenna. Second, it reduces the amount of
inductance you need and therefore for a given Q reduces the amount of
inductor loss. I think you'll have trouble getting reasonable Q and
therefore reasonably low loss from a small, multilayer air wound coil.
You're best off with a single layer air core solenoid or, not quite as
good but decent if done with care, a single layer winding on a large
type 2 or 6 powdered iron core.

Just remember to protect the inductors to keep water from between the
turns. Also, remember that just beyond the inductors the impedance is
pretty high, so keep the part of the antenna beyond the inductors, in
particular, away from leaves, branches, and the like, especially if
they're wet.



Thanks, Roy. Since my antenna will be very portable, with a
collapsable fibreglass mast I can't take much weight, ergo, the need for
small wire, and light weight inductors. However, it appears I would be
better off with feedpoint inductors instead of putting them halfway out
the legs of the dipole. I will plan on doing that, and will look into
using iron core inductors instead of air wound.... if I can keep it light
enough. (the mast sags under any appreciable weight). Thanks again.

Ed K7AAT

Ed, K7AAT wrote:
"I plan on building a short dipole (inverted V) for traveling in my tiny
RV. It will be designed for 75M."

A 70 microhenry, or any inductor, is lossy. Why not make the RV part of
a small loop which can be tuned with a low-loss capacitor?

My father-in-law was W1DBM, now SK (born 6-6-6) who did the RV thing and
ham radio most of his life. He found his Airstream loop was 15 to 32 dB
better than a trailer dipole or whip.

W1DBM`s loop appears in a 1978 ARRL "Antenna Anthology"

Best regards, Richard Harrison, KB5WZI

"I plan on building a short dipole (inverted V) for traveling in my
tiny
RV. It will be designed for 75M."


A 70 microhenry, or any inductor, is lossy. Why not make the RV part of
a small loop which can be tuned with a low-loss capacitor?

My father-in-law was W1DBM, now SK (born 6-6-6) who did the RV thing
and
ham radio most of his life. He found his Airstream loop was 15 to 32 dB
better than a trailer dipole or whip.

W1DBM`s loop appears in a 1978 ARRL "Antenna Anthology"

Best regards, Richard Harrison, KB5WZI


I'll have to look that up, but I don't think a loop is a viable
option. For one thing, my trailer is only 17' long body. Plus, there is
no way I can get on the roof of this small trailer without dragging
around a ladder. Also, I expect to want to use this antenna when the
trailer is not available, such as possible tent camping.

Thanks for the input, though.

Ed K7AAT

However, it appears I would be
better off with feedpoint inductors instead of putting them halfway out
the legs of the dipole.

Myself, I would prefer them out at 1/2 or even 2/3 from the center.
Better current distribution. What will the antenna be made from?
Wire, or metal tubing? Regular solenoid coils can be made from
any light plastic tube, using light wire. I use insulated wire to act
as a spacer. I then cover them with shrink wrap, or tape.
If the antenna is hard to match to coax, you can use a third coil
at the feedpoint, and tap it for the best match.
If the antenna was tubing, I would use wooden dowel that would
fit in between a section of metal tubing, and wind the coils on those.
I'd weatherproof those some way also.
When I make small loaded dipoles, I think exactly the same way as
I would designing a mobile whip. I'd make the most efficient design
of a 24 ft whip, IE: coil placement, etc, and just double it for a
dipole.
Whats best for a short whip, will also be best for a short dipole
of the same size per leg. The very best location is probably at
the 2/3 to 3/4 mark, but it's not that critical. Even 1/2 out is good.
Reg's vertload can model all that, and tell you the best place, coil
info, etc, etc... In comparing to the real world, it's close nuff for
gov
work. If the apex was high, even a center load could be ok, but if
you only have 5 watts, I would go for every last scrap of efficiency
in the design. Actually, I'd also carry a spare 120 ft of wire to use
as
a full size dipole in the off chance the location could support it. :/
Many would, if they have trees to hang wires from, etc.. Even if the
full dipole is perverted, and making turns, a "Z",etc, it will usually
outdo
the loaded antenna for 75m.
MK

Just a followup on my original post under this subject.

I completed my short 75M dipole (Inverted V) and am very pleased with
it.


Went to the hardware store, bought a piece of cheap bathroom plumbing
material, 1.5" diameter and very light. (Lighter than PVC)

Bought some #18 stranded hookup wire.

Found an old spool of Telephone indoor station wire (#22).


I wound my 1.5" dia coils with the telephone station wire. Worked
very well. I wanted a 50 uH coil so as it turned out, 50 turns, close
spaced, did the job. (about 25 turns per inch).


I positioned the coils about 6/10 of the distance down my dipole
legs; the total length of each leg came out to 24 feet. The feedpoint
is at the top of my 23 foot cheap telescopic RV flag pole. This antenna
tuned right up to freq... 3950KHz in my case. SWR was about 1.6. Seems
usable between 3900 and 4000. If I want to go lower, I just hang a
couple alligator clips with 6" wire off the ends to lower resonance.

Project was easy, and whole thing is light, small, easy to erect, and
works well, considering size. Works well with my FT-817.

Thanks to those who made replies to my post.


Ed K7AAT