From: "Bill" <w7vp@comcast.net>
To: "Jim Lux" <jimlux@earthlink.net>
Subject: Re: [TowerTalk] mismatch at feed, ok at transmiter
Date: Saturday, April 15, 2006 7:35 AM
Jim
Nice explanation
Feedpoint matching is a big problem for most of us amateur designers. I
have had a big 20 meter yagi for more than 30 years. It came as a Bill Orr
type design with 4 elements equally spaced on a 36 foot boom and a T-match.
When YA came out I redesigned it an put on the 5th element. In doing so I
split the driven element to feed it with a 1:1 balun
It turned out that the feedpoint impedance was in the range of 20-j25 ohms.
I shortened the driven element a bit to reduce the capacitive reactance and
fed it with two pieces of 75 ohm coax in parallel cut to a quarter
wavelength and connected to the amp (a 4-1000A) with 75 ohm feedline. The
quarter wavelength matching section was almost perfect as the geometric mean
of the feedpoint and the feedline.
I just put the antenna backup last week after having it down for almost
three years and I am always astounded how good it is. It is at least 3
s-units better than my Swan tribander on a separate tower at the same
height.
If anyone is interested I will give more details. On thing of note is that
the elements are large at 1-1/2" for the 12 foot center sections
(6"half-length) with the remainder of the element nested into the center
section at 1-1/4" with a matching sleeve. The fittings are cast aluminum
with no marking identifying the original manufacturer and the boom is a 2"
square section turned with the corner up. The mast mounting is also cast
aluminum with a lug and clevis arrangement that allows the boom tow be swung
down the tower for mounting and dismounting the elements. (Makes erection
fairly easy for such a big beam). I would love to hear from anyone who
thinks they know who made it originally.
Bill
W7VP
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Friday, April 14, 2006 7:06 AM
Subject: [TowerTalk] mismatch at feed, ok at transmiter
> There are antennas which will appear to be a poor match at the
> feedpoint. For instance, a helical antenna might have a 100 ohm feedpoint
> Z. A common strategy is to use a 1/4 wave transformer of 75 ohm coax to
> get the impedance closer to 50 ohms. (56 ohms in this case). The loss
> from
> the relatively short section isn't all that high.
>
> Likewise, in phased arrays, there are all kinds of schemes around where
> the
> elements won't be anywhere near 50 ohms at the feedpoint.
>
> In a single feedpoint antenna, it might just be a design decision to let
> the Z at the feedpoint be non-50 ohms, and tolerate the extra loss in the
> feedline. After all, how many people string up dipoles, which have a
> feedpoint Z of around 70 ohms, and just accept the 1.4:1 SWR (with 200 ft
> of RG-213, it drops to around 1.35:1 on 40m)
>
> For small mismatches (2:1 or less), the extra loss from the circulating
> power just isn't all that huge. It may well be that trying to put a
> matching network at the feedpoint might make the antenna system have more
> loss or (more likely) be narrower band. The antenna design typically
> assumes a broadband resistive 50 ohm termination when calculating all the
> mutual interactions. If you put some sort of reactive matching network,
> the performance may be improved at one frequency at the expense of many
> others.
>
> (The extreme in this might be the Terminated Folded Dipole.. a
> deliberately
> lossy antenna, that presents a reasonably good match over almost the whole
> HF spectrum)
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Friday, April 14, 2006 7:06 AM
Subject: [TowerTalk] mismatch at feed, ok at transmiter
> There are antennas which will appear to be a poor match at the
> feedpoint. For instance, a helical antenna might have a 100 ohm feedpoint
> Z. A common strategy is to use a 1/4 wave transformer of 75 ohm coax to
> get the impedance closer to 50 ohms. (56 ohms in this case). The loss
> from
> the relatively short section isn't all that high.
>
> Likewise, in phased arrays, there are all kinds of schemes around where
> the
> elements won't be anywhere near 50 ohms at the feedpoint.
>
> In a single feedpoint antenna, it might just be a design decision to let
> the Z at the feedpoint be non-50 ohms, and tolerate the extra loss in the
> feedline. After all, how many people string up dipoles, which have a
> feedpoint Z of around 70 ohms, and just accept the 1.4:1 SWR (with 200 ft
> of RG-213, it drops to around 1.35:1 on 40m)
>
> For small mismatches (2:1 or less), the extra loss from the circulating
> power just isn't all that huge. It may well be that trying to put a
> matching network at the feedpoint might make the antenna system have more
> loss or (more likely) be narrower band. The antenna design typically
> assumes a broadband resistive 50 ohm termination when calculating all the
> mutual interactions. If you put some sort of reactive matching network,
> the performance may be improved at one frequency at the expense of many
> others.
>
> (The extreme in this might be the Terminated Folded Dipole.. a
> deliberately
> lossy antenna, that presents a reasonably good match over almost the whole
> HF spectrum)
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
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