title: The feedline antenna
slug: feedline-antenna
date: 2026-08-25 12:55:12 UTC+02:00
modified: 2026-08-25 17:10:23 UTC+02:00
type: text

## Summary [🔗](#summary){.small} {: #summary}

- A simple antenna that is built from a length of 300 Ω twin-wire
  feedline, and some short piece of plastics.
- Budget antenna! The coax feedline you'll attach to it will most
  likely be more expensive than the antenna itself.
- Mostly meant for 2 m, but can be used for 70 cm as well.
- Very light, so you can get it up high into the air on a flimsy
  telescope mast.
- But it has a few peculiarities you should be aware of.

## What kind of an antenna is this? [🔗](#analysis){.small} {: #analysis}

This is a variant of the J-pole antenna.

Simultaneously, it is also a end-fed half wave dipole, fed via a
quarter-wave stub.  The upper 97.5 cm of the antenna form that dipole,
the rest is the quarter-wave stub that feeds it.

As far as radiation and gain are concerned, this antenna performs like
any (vertical) half-wave dipole does.

## Material [🔗](#material){.small} {: #material}

The following instructions are for 300 Ω feedline with a velocity
factor of 0.8.  This is the line that has no windows.

The tough part is to obtain such 300 Ω feedline these days.  Honestly,
I do not know a source right now.

I myself bought mine from [DX-wire](https://www.dx-wire.de/) when it
was carried there, maybe some 12 years ago.  Despite the cheesy name,
DX-wire is a good, honest shop, the owner Peter is quite helpful, and
I recommend that shop and especially their rods.  I've been a happy
customer of DX-wire's products for at least 15 years, but am not
affiliated or otherwise connected with it.
{: .deemphasis}

Another source with a good reputation I'm not hesitant to recommend is
Kabel Kusch.  They presently have [windowed 300 Ω
feedline](https://www.kabel-kusch.de/produkt/cq562/12) that I expect
to be quite usable for this kind of antenna.  But as windowed line has
a different velocity factor, the sizes given here will not work for
that feedline; you will need a longer stub.  Some remarks
[below](#other_feedlines) may help you to get started.
{: .deemphasis}

## Building instructions [🔗](#building){.small} {: #building}

The basic building instructions in German are contained in this
sketch.  The same information, with some more detail, follows as
English text.

![A sketch of the antenna, with measurements.  You can take the text that follows as an image description.](/images/2026/feedline_antenna.png)
{: .small}

You start with a piece of feedline that's about 142 cm long or longer.

Call one end the lower and the other the higher end.

Start with short-circuiting the lower end.

Next, 2.8 cm from the lower end, you attach a coax to the feedline.
The coax screen is soldered to one conductor of the feedline, the
inner conductor to the other.  You'll have to expose the conductors
over a length of a mm or two, without cutting them.  A sharp knife, a
cutting board, and a bit of patience generally do the trick.

The sketch is a bit symbolic here.  The lengths of the two coax
conductors to their respective solder joints should be as short as you
can make them.  The length of the coax should extend downwards, in
line with the feedline.

If you goof on the first try, but you have enough feedline left, you
can cut off the bottom 3 cm or so and start over.  Short circuit the
lower end, measure 2.8 cm, attach the coax.

Bend the 2.8 cm feedline length so it sticks a bit away from the coax.
A 30° or 45° angle or so is enough.

Next, you need to cut your feedline to length.  You need to end up
with a total length, from one end to the other, of 140.8 cm.  But the
upper end needs to be short-circuited as well!  So cut a little
longer, adding the width of the feedline to the 140.8 cm.  Then remove
the inner material and strip the two conductors over such a length
that the stripped conductors start 140.8 cm from the short circuit on
the other end.

Short circuit the upper end.

Finally, a total of 41.3 cm from the lower end, you cut one of the two
conductors of the feedline.  It does matter which one: You need to cut
the one that is soldered to the coax screen.

Another 2 cm above that cut, you need to cut that same feedline
conductor again.  Then remove those 2 cm of that conductor.

Done.  The electric part of the antenna is completed.

If all is well, the feedline will show SWR better than 1.4 over the
entire 144-146 MHz range, *if mounted correctly.*  Mounting is somewhat
critical, see below.  For tuning hints, see below, too.

There is some suspicion that the lengths as given here are maybe a bit
long.  If you dare to, you can try a stub length of 41.0 cm and a
total length of 140.0 cm.
{: .deemphasis}

This antenna can also be used on 70 cm.

## Mechanical support for the solder joints [🔗](#de-stress){.small} {: #de-stress}

The solder joints between the coax and the feedline will break
eventually if subjected to even moderate mechanical stress.

Attach some reasonably stiff stick of plastic to the coax below and
also to the feedline above the solder joints.  Fortunately, this is
not a fiddly spot, so you can pretty much use what you have.  A
literal stick of wood would probably work, too.

The idea is: If you pull at the feedline and the coax, none of that
force reaches the solder joints.

For the attachment of the stick to the antenna, I personally used
stone age technology: Rope wrapped around both.  That is, a piece of
thin plastic rope tied some ten times quite firmly around both coax
and my plastic stick (I used a piece of thin plastic tube maybe 10 or
15 cm long), and another such around the feedline and my plastic
stick.  Cable ties would also work.

## Mounting is a bit fiddly. [🔗](#fiddly){.small} {: #fiddly}

This feedline antenna is an end-fed half-wave antenna.  Like many such
antennas, it is somewhat fiddly, does not like stuff near the dipole
ends.

Where are the dipole ends? That is, where are the fiddly spots? The
upper end of the antenna is one, and the general vicinity of the gap,
that is, the place where one of the conductors is cut, the other.

**Do not use adhesive tape or heat-shrink tube over the 2 cm gap to
waterproof it.**  It will cause the frequency of the antenna to drop
considerably.

Also, try to keep some distance of 5, better 10 cm between those two
fiddly spots and whatever rod you use to lift it up.  The rod should
be fiberglass, or some other plastics.  Do not use carbon fiber!

Personally, I stick my rod out of the window.  Therefore it does not
go straight up, but at an angle. But I want the antenna straight
vertical.  So I tie it to the rod tip with a piece of rubber string
and let it dangle a bit.

A remark on those rubber strings: I cut them from used bicycle inner
tube, which I get from my local bike shop for the asking.  The strips
are about 20-30 cm long, 1 cm wide.  I use these rubber strings a lot
to attach stuff.
{: .deemphasis}

But I do not tie the very upper end of the antenna. I let that stick
up 10 cm above the rod's end, so tie the antenna 10 cm down to the tip
of the rod.  After all, the antenna's tip is one of the two fiddly
spots, so I want to keep that out of harm's way!  (If that 10 cm bend
a bit, I'll rotate the rod so the bending points mostly upwards.  But
that probably doesn't matter much.)

Suspended this way, the antenna dangles down.  That is just as well,
as it keeps the second fiddly spot (where the 2 cm gap is) away from
the rod.  I measure out a length of coax and tie that to the rod with
another rubber, about as high as the lower end of the antenna dangles.
This way, the coax forms a little horizontal bow from the lower
antenna end to the rod. There's slack, the antenna itself can still
dangle freely.

I then lead the coax down the rod. I fix it to the rod with yet
another of my rubber strings every 2 m or so.

That's what I do out of my fourth-floor window.

If I'm elsewhere, I may need to attach this antenna to a rod that's
vertical.  I'll again let it stick out over the top of the rod by some
10 cm, tie it to the top there, and then tie it a little above the
coax so that it bulges slightly.  A distance of some 5 cm, better 10
cm between mast and the fiddly gap spot is healthy.  If I need to, I
can tie the antenna with another rubber strip some 40-50 cm from its
top end.  That comes out as the middle of the dipole; the antenna is
quite non-fiddly there.

## Tuning [🔗](#tuning){.small} {: #tuning}

If your antenna's best SWR is at a frequency much lower than the band
center at 145.000, you can try to tune it.

We tried that yesterday, when an antenna built for a friend to the
above specifications was maybe a MHz low in frequency.

We have found out the hard way that simply removing some length from
the upper end has less than the desired effect.

Naively, one would think: This is a half-wave dipole at 145 MHz, so
removing 0.67 mm should lift the frequency up by 100 kHz.

But in practice, removing that length actually seems to do only
roughly half of the expected lifting in frequency.

But after a lengthy process of several rounds of trial and error, we
still finally got the frequency good enough yesterday.

Only afterwards, it dawned on me: For each mm that we shortened the
antenna at the upper end, we should have also cut half a mm from the
stub wire that is connected to the coax shield, making the gap
slightly longer than its initial 2 cm.  We didn't actually try that.
But you may.

## Waterproving [🔗](#waterproving){.small} {: #waterproving}

Adhesive tape is fine over the joins to the coax, but do not use it
over the gap and preferably not over the upper end of the antenna.

Still, it is a worthy goal to seal up everything so water will not
creep into the wires or the coax.  I personally smeared some glue over
the wires.  Not sure how well that actually worked, but maybe it did.

But even if the goal is achieved, the antenna still prefers to be
operated in dry weather.  If you have appreciable moisture build-up in
the region of the gap, SWR will suffer.

I do not use my antenna in a permanent setting, but stick the rod out
of the window when I want to operate and pull it back in afterwards.
Not being able to shut the window poses somewhat of a challenge in
both stifling climate catastrophe-induced summer heat and winter snow
drizzle, but it is fine during the more moderate seasons on dry days.

That said, the antenna has been giving me a lot of faithful service.
I have been using it in our weekly local FM round for at least 11½
years.

## Maybe other feedlines? [🔗](#other_feedlines){.small} {: #other_feedlines}

You should be able to build the antenna with 300 Ω window line, or
even with the also windowed more popular 450 Ω feedline, but then
you'd have to experiment with the sizes.  See to it you get feedline
that is constructed from *stranded* wire as the two conductors.  Some
cheap offers give you solid wire; I'd be afraid that such wire will
eventually break.

If I wanted to do that, I'd start with determining the length of a
quarter wave stub at 145 MHz.  For that, I'd attach a resistor of 1.8
kΩ at the other end of 300 Ω line, or 4.05 kΩ at the other end of 450
Ω line.  These are the values a quarter-wave stub of that feedline
will transform to 50 Ω.

So I'd try to find the length where SWR is minimal at 145 MHz.  That
only needs a trx that can measure SWR at low power, and resistors that
can take that low power for a few seconds at a time.  If you have
sophisticated measurement instruments, you'll probably find an (even)
easier way.

That would be what I'd use in place of the 41.3 cm.  The remaining
97.5 cm above the gap I would leave alone initially, regardless of
feed line used.

## Discussion opportunity [🔗](#discuss){.small} {: #discuss}

If you want to comment or discuss this piece and have a Fediverse
account, feel invited to answer [my pertinent
toot](https://mastodon.radio/@dj3ei/117155909589032263).