Vetical Antennas

[James Duffey]

Mike - You have already received some good advice on your 5BTV  
installation. Let me add my bits of advice.

1. If you use a lot of radials in an elevated installation, they  
don't need to be resonant. The purpose of the radials is to  provide  
a return current for the transmitter and screen the antenna from the  
ground (in your case the roof and house) to reduce losses. So I  
suggest that you start with 16 or more radials the height of the  
vertical. A conventional ground plane antenna does not really act  
like a ground plane antenna unless it is fairly high above the ground.

2. A choke balun at the antenna feedpoint, one where it exits the  
radial field, and one at or near the transmitter should help keep the  
common mode currents down. A good choke balun can be made by winding  
as many turns of the feedline around a FT240-433 or -61 ferrite core  
as you can manage. There are other solutions that will work, such as  
ferrite beads or an air core balun, but the ferrite toroid is simple  
and effective.

3. Some commercial verticals are "DC grounded" by design with an RF  
choke across the feedpoint. Check to see if the 5BTV is by using an  
Ohm meter across the coax connector. If you get continuity you are  
probably OK. If you can add such a choke use 2 to 3 microHenrys. If  
this is the case, you are in good shape, add a lightning protector,  
like the Polyphaser or ICE where the coax enters the house and  
install a good ground. This is a good place to add the choke balun,  
put it after the lightning protector.

4. The admonition against the tuner is to prevent you from using the  
antenna when it is not functioning properly or from using it on a  
band for which it is not intended. Using it within the band is OK.

5.Moxon's concerns about quarter wave radials stem from two points.

        a. If the quarter wave length radials are not exactly the same, then  
there can be considerable disparities in the current each radial  
carries, along with high angle horizontal radiation and reduced  
radial efficiency. Shorter or longer radials share currently more  
evenly even if they are not exactly matched in length. Longer radials  
can be tuned to resonance with a series capacitor.

        b. Quarter wave radials are more likely to couple to the feed line  
causing common mode currents on the outside of the coax. Using a  
choke balun at several points on the feedline, as indicated above,  
will help.

You will avoid problems in radials by increasing the number of  
radials you use. This will reduce the magnitude of current in each  
radial and spread the current over a greater area, both important  
factors in reducing ground losses and increasing efficiency.

6. Don't expect very good performance on 80M. The antenna is short  
and you are unlikely to be able to put in a good enough ground system  
to make it very efficient. Still, it is better than no antenna.

7. DX Engineering has a 5BTV manual on their website that may have  
more information on it than you currently have. Check it out.

Let us know how it all works out. - Dr. Megacycle KK6MC/5


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[David Cutter]

Dear Doc

Given the 5BTV and similar trap verticals have reduced performance on 80m
and nil on 160m, do you have a scheme for using the whole elevated system as
the antenna on these low bands, eg a tuning/switch box in the support pole?

David
G3UNA

----- Original Message -----
From: "James Duffey" <[hidden email]>
To: <[hidden email]>
Cc: <[hidden email]>; "James Duffey" <[hidden email]>
Sent: Saturday, March 03, 2007 5:04 AM
Subject: [Elecraft] Vetical Antennas

> Mike - You have already received some good advice on your 5BTV
> installation. Let me add my bits of advice.
>
> 1. If you use a lot of radials in an elevated installation, they  don't
> need to be resonant. The purpose of the radials is to  provide  a return
> current for the transmitter and screen the antenna from the  ground (in
> your case the roof and house) to reduce losses. So I  suggest that you
> start with 16 or more radials the height of the  vertical. A conventional
> ground plane antenna does not really act  like a ground plane antenna
> unless it is fairly high above the ground.
>
> 2. A choke balun at the antenna feedpoint, one where it exits the  radial
> field, and one at or near the transmitter should help keep the  common
> mode currents down. A good choke balun can be made by winding  as many
> turns of the feedline around a FT240-433 or -61 ferrite core  as you can
> manage. There are other solutions that will work, such as  ferrite beads
> or an air core balun, but the ferrite toroid is simple  and effective.
>
> 3. Some commercial verticals are "DC grounded" by design with an RF  choke
> across the feedpoint. Check to see if the 5BTV is by using an  Ohm meter
> across the coax connector. If you get continuity you are  probably OK. If
> you can add such a choke use 2 to 3 microHenrys. If  this is the case, you
> are in good shape, add a lightning protector,  like the Polyphaser or ICE
> where the coax enters the house and  install a good ground. This is a good
> place to add the choke balun,  put it after the lightning protector.
>
> 4. The admonition against the tuner is to prevent you from using the
> antenna when it is not functioning properly or from using it on a  band
> for which it is not intended. Using it within the band is OK.
>
> 5.Moxon's concerns about quarter wave radials stem from two points.
>
> a. If the quarter wave length radials are not exactly the same, then
> there can be considerable disparities in the current each radial  carries,
> along with high angle horizontal radiation and reduced  radial efficiency.
> Shorter or longer radials share currently more  evenly even if they are
> not exactly matched in length. Longer radials  can be tuned to resonance
> with a series capacitor.
>
> b. Quarter wave radials are more likely to couple to the feed line
> causing common mode currents on the outside of the coax. Using a  choke
> balun at several points on the feedline, as indicated above,  will help.
>
> You will avoid problems in radials by increasing the number of  radials
> you use. This will reduce the magnitude of current in each  radial and
> spread the current over a greater area, both important  factors in
> reducing ground losses and increasing efficiency.
>
> 6. Don't expect very good performance on 80M. The antenna is short  and
> you are unlikely to be able to put in a good enough ground system  to make
> it very efficient. Still, it is better than no antenna.
>
> 7. DX Engineering has a 5BTV manual on their website that may have  more
> information on it than you currently have. Check it out.
>
> Let us know how it all works out. - Dr. Megacycle KK6MC/5
>
>
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[James Duffey]

Dave - I sure don't have anything specific off the top of my head.

You might try replacing the resonator coil and the whip above that  
for 80 M with a 40 M trap and extending a wire long enough to make  
the antenna resonant on 80 M from the top of the trap horizontally to  
a tree or something. You could even droop it a bit/ This would  
improve the efficiency on 80 M a lot over the stock arrangement. If  
you had enough horizontal space, you might put a 160M trap at the end  
of that wire and add enough wire to make that resonant at 160M.

Both of these, while not great efficiency wise, would at least get  
you on the band. - Dr. Megacycle KK6MC/

--
James Duffey KK6MC/5
Cedar Crest NM 87008
DM65

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[David Cutter]

Doc

I didn't mean to mislead you: I don't own a 5BTV myself, but there seems to
be a lot of them around.  If  it is common practice to put them on a pole,
then the pole ought to be useful on the lower bands using the 5BTV and its
radials as at least partial loading for 80 and 160.

A simple idea might be to have a separate feeder for the lower bands to the
bottom of the pole with a loading/matching coil, though it would need a lot
of wire in/on the ground to make it half decent.  At least the higher bands
would be relatively efficient.

But I can't help thinking that feeding the pole from the top would be
workable and perhaps more efficient: I've seen this done with a beam and
tower arrangement in which the beam is isolated from the tower and an auto
tuner fitted between.

David
G3UNA


----- Original Message -----
From: "James Duffey" <[hidden email]>
To: "David Cutter" <[hidden email]>
Cc: "James Duffey" <[hidden email]>; <[hidden email]>
Sent: Saturday, March 03, 2007 7:58 PM
Subject: Re: [Elecraft] Vetical Antennas

> Dave - I sure don't have anything specific off the top of my head.
>
> You might try replacing the resonator coil and the whip above that  for 80
> M with a 40 M trap and extending a wire long enough to make  the antenna
> resonant on 80 M from the top of the trap horizontally to  a tree or
> something. You could even droop it a bit/ This would  improve the
> efficiency on 80 M a lot over the stock arrangement. If  you had enough
> horizontal space, you might put a 160M trap at the end  of that wire and
> add enough wire to make that resonant at 160M.
>
> Both of these, while not great efficiency wise, would at least get  you on
> the band. - Dr. Megacycle KK6MC/
>
> --
> James Duffey KK6MC/5
> Cedar Crest NM 87008
> DM65
>

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[James Duffey]

Dave - You Wrote:

"But I can't help thinking that feeding the pole from the top would  
be workable and perhaps more efficient: I've seen this done with a  
beam and tower arrangement in which the beam is isolated from the  
tower and an auto tuner fitted between."

Yes that would work. I was not quite envisioning what you were  
asking. a relay to short the vertical to the radial field and then  
feeding that against the pole would work, perhaps with some added  
capacitance or inductance to make it all come out in the band you  
want. - Dr. Megacycle KK6MC/5









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[Stuart Rohre]

I have toyed with adding 160m to an existing "80 to 10m band" vertical by
the simple expedient of placing an 80m trap at the top of the vertical, then
leading off a wire to make an L antenna with enough horizontal length to
load the whole on 160m.

Remember, you get nearly the efficiency of a dipole with 60 per cent of its
length.  That implies that if you had 30 per cent of a dipole for 160, and
added that to your vertical, you should have reasonably efficiency on 160.
(with a proper radial system).  Now this is an L operated against ground,
and likely it would benefit from as many radials as you can place at the
base of the vertical section.   Many short radials are better than too few
long ones, and at 160m, about the only types the average ham can manage will
be short radials.

Even better, is if you could rig your vertical to lay over for attaching the
160m wire extension, and then take it back off when your interests lie with
the higher bands.  That reduces the add on to only a wire, no trap needed.
Tilt mounts are available for the Gap verticals, and a similar home made
mount could be fabricated from angle irons from the local home center store,
or hardware store.

Stuart
K5KVH


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