[TowerTalk] Choke on feed point of dipole

[Jim Lux]

Yeah, the pattern doesn't change much with the physical length of the dipole/monopole, if it's < 1/2 wavelength.
There's a slight difference in "efficiency" (defined as how much power gets radiated vs how much is in the feedline) - smaller antennas have higher currents, so more IR loss, and tend to be narrower BW or need matching networks (which might be lossy).

And "how high it is off the ground" makes a much bigger difference.  Both in the pattern (because of the reflected wave) and in efficiency (because the field heats up the soil, or not).

You could do a tradeoff of elevated vertical dipole, elevated 1/4wave with some counterpoise, vs closer to the ground.  Flooding your yard (and neighbors') with salt water would almost certainly help, but may have other non-radio side effects.

Too bad that V pol is such a pain to work with - HFTA with vertical pol would be a useful tool.
 


On Tue, 13 Jan 2026 12:09:02 -0700, David Gilbert <ab7echo at gmail.com> wrote:

"Assuming you have the height"

That's the kicker, though, and it takes twice as much of it for
relatively little additional performance.  I've modeled a 20m ground
plane with four elevated radials and a 20m vertical dipole, both of them
being 4 feet off the ground.  The elevation pattern is lower with the
vertical dipole (17 degrees versus 23), but the maximum gain is almost
identical.

I agree that your suggestion (feedpoint at the bottom with a serious
choke) is a practical way to make a vertical dipole, but you have to
trim for tuning at the top whereas you can trim a ground plane at the
radials near the ground.

73,
Dave   AB7E


On 1/13/2026 11:35 AM, Jeff Blaine wrote:
> I would not hesitate to put up a vertical dipole.
>
> In fact my 30m beam is so directional I'm thinking of putting up one
> here just to cut down on rotor wear & tear.  Assuming you have the
> height, It's much simpler physically than doing a vertical as you
> don't have the ground system to worry about.  And it gets the
> important current maximum quite a way off the ground.
>
> The "easy" way to do this is to build the dipole as normal. **NO**
> choke at the feepoint in this case.  The **GROUND** leg of the dipole
> runs down to ground, next to the coax with the two secured together so
> they are not blowing around separately.
>
> The important part is to put a serious coax choke at the end of the
> dipole tip, on the ground side.  That means multiple turns through a
> couple of type-31 ferrites.
>
> Antenna resonance is trimmed with the tip length at the top assuming
> you have a pulley to pull it up.
>
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
>
> On 1/13/2026 4:02 AM, Brian Beezley wrote:
>> Tom Hellem wrote:
>>
>> "I think the reasonable conclusion is that a center fed vertical
>> dipole is a very difficult thing to make work..."
>>
>> Tom, at my last QTH I dropped a 40m dipole vertically from a tall
>> eucalyptus. I fed it directly with RG-58 (no choke). The feedline ran
>> roughly horizontal for tens of feet. (The tree was slightly down the
>> slope of a hill from the shack.) SWR was fine. I remember generating
>> pileups during Field Day as a 500 watt home station, but otherwise I
>> was not that impressed with its performance.
>>
>> The gain and elevation pattern of a vertical antenna are quite
>> sensitive to ground quality. Unless you have really good ground, a
>> horizontal antenna may perform better, even at low angles, if you can
>> put it at a decent height. "Decent" might not be that difficult at 14
>> MHz and above, but it may be a problem below.
>>
>> When modeling a vertical antenna, these generic ground constants are
>> much more appropriate than those your antenna analysis program offers:
>>
>> https://k6sti.neocities.org/hfgc
>>
>> Brian
>>
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