たまたまWebを見ていたら同軸スタブフィルタ作成についての過去のQ&Aをまとめたものを見つけましたので英語ですが備忘に載せておきます
各人のコメントはどこの水準までこのフィルタに求めるのかによって答えが変わってくるようですが、総じて言えるのは以下のようです
・同軸は出力に対して極端に細いものでなければ利用可能
ただし、フィルタの減衰および急峻性は同軸の減衰性能に依存するので可能であれば
できるだけ性能の良い同軸を使うのがよい
・７５Ω系の同軸の利用も実用範囲のようですが、調整はVNAなどを使ってしっかりする
必要があり
・アンテナ切替機などをかます場合は、コネクタなど追加するものを含めて調整する必要
がある
・CWとSSBについて同じフィルタで使えるのかについても、こだわるのであれば別々に
作る方がよいとの意見有
実際、TOP TEN DEVICEが同軸スタブフィルタを商品化していますが、注文の際には
CWかSSBを指定するようになっています
・よりよい成果を得るためにはトランシーバーとリニアの間にバンドパスフィルターをかまし
リニアの後にスタブフィルターを入れるのがベター
DH1TWによればED1Rでは4O3A製のハイパワーバンドパスフィルターをリニアの後ろに
入れているが、さらに同軸スタブフィルタを追加することで急峻特性が増し、更に
３０ｄｂの減衰が可能になったとのこと
https://www.dh1tw.de/will-coax-stubs-improve-bandpass-filter-performance/
coax stub summary
To: cq-contest@tgv.com
A while ago I was looking for info on making/using coax stubs for
filtering between two stations. Thanks to W1PH, VS6WO, KY1H,
K3NA, K1VR, W0CP, K3LR, KR0Y, WA6OTU, WM2C, ON6TT, N4OGW,
(and maybe a few others who got lost while cutting and pasting
this together) for the hints and comments. Several cited the K2TR
article in the May - June 1984 NCJ. Following is a collection of
the various comments received (in no particular order) beginning
with the original post.
Mike N0BSH
n0bsh@aol.com
-----------------------------------------------------------
I'm looking for information on using coax stubs for filtering
between the two stations. Some questions that come to mind
include:
1) Lengths required for nulling various bands - open/shorted?
2) Can stuff like RG58 be used or should we stick with RG8 (will
be running about 1KW on each station?)
3) Do they need to be made with 50 ohm coax or can 75 ohm be
used (have some of both laying around?)
4) Can we put these on a coax switch and switch in the ones we
need depending on band?
5) Is it worth all this or should we just get out the MC/Visa and
buy the Dunestar filters?
-----------------------------------------------------------
1. Lengths required: This depends on the velocity factor of the
coax being used to make the stub. In addition, you will have to
have an appropriate serial coax impedance transformer between
the point where the stub and antenna feed attach and the radio.
Otherwise the radio will see a wierd impedance and potentially
high SWR. The lengths of the serial matching coax sections will
be different for each combination of antenna and band-stub. (If
you are getting the impression that this is complicated, you are
correct!) Either shorted or open-ended stubs can be used.
However, shorted stubs are easier to build properly. Open stubs
tend to leak at high powers (remember, that will be a very high
voltage point at the end of the open stub!)
2. The quality of the stub as a filter depends on the loss of the
coax. RG58 coax has high losses and you will lose many 10s of dBs
of filtering. Even RG-213 is pretty marginal... (This is another
problem for the use of stubs in an expedition...)
3. Effective stubs can be designed using any impedance of cable,
although the lengths of the matching sections will change.
4. Coax switching of stubs is very complicated because the
matching sections must also be switched.
5. It is simpler to buy the filters. You will get similar
performance to reasonable-quality stubs (altho not as good as
excellent stubs, and not at 1500 W power levels). Switching is a
breeze. And Dunestar or ICE filters are smaller and lighter to
carry with you to the site.
-----------------------------------------------------------
The stubs are easy and cheap. I use RG62A/U that I bought by the
pound from a metal salvage yard. Impedance of the stubs does not
have to match the transmission line. RG62A/U is 93 ohm coaxial
cable. Small cable is fine as it only has to carry the spurious
signal. I use 6 dedicated feed lines for the 6 bands and have stubs
permanently attached to each line. There is no reason not to use a
switch to select the right stub. But if you forget to switch, the
amplifier may be looking at a short circuit with the wrong stub.
Stubs are not real high Q devices and seem to cover an entire band
easily.
-----------------------------------------------------------
An effective combo of stubs and filters allows almost any
band/antenna combo to be on the air at the same time.
-----------------------------------------------------------
Yes, stubs are worth it even if you use ICE or other tx filters I
think. Most of the other filters are only exciter level so you put
them between the exciter and amp, but they don't do anything for
amp produced harmonics. In general the stubs are 1/4 wave
shorted pieces of coax, except on 40m to null out 15m harmonics.
If you are running kw's you want to use at least rg8x, preferably
rg8 or rg-213. We melted down a stub using rg-59 here one
contest. No, the impedance of the coax doesn't matter, but the
lossiness of it does. Going from rg-58 to 3/4" hardline increases
nulls from about 15 to about 40db. Using decent quality rg-8 gives
good results. Yes, you can switch stubs, just remember that the
size of the switch and any connecting coax pieces add to the
length of the stub so shorten up the stub to compensate for it.
-----------------------------------------------------------
As most expeditions involve dragging too much stuff through
reluctant airline channels, I'd recommend buying the DuneStars.
Yes, you may bandswitch stubs in and out. Yes, you may make them
out of 75 ohm cable. But to make stubs that work, you MUST have
a testing machine that will read out frequency (AEA, MFJ, network
analyzer, etc.) RG-58 vs. RG-213: The less loss, the better the
stub performance. Remember, DuneStars go between xcvr and amp.
Stubs of RG-213 can go after amp. I use ICE bn xcvr and amp,
stubs of .750 inch diameter between amp and antenna.
-----------------------------------------------------------
Coax stubs are best at reducing broad band phase noise from
nearby transmitters. They will provide about 20-25db attenuation
on the next nearest band. The lower the loss of the coax used on
the stub, the deeper and narrower the notch will be - take your
pick from any coax that will handle the power. Note that stations
have had stubs melt in the 'heat' of battle!. Better stick with 50
ohms... Many stations don't bother with stubs in multi-single -
just good rx filters.
-----------------------------------------------------------
There is a middle way, which is to use an ant tuner as a "tunable
stub". This doesn't work in a m/m operation, but for what you guys
are doing, it might be the best approach. I have used MFJ 16010
tuners for this, which cost about $30. I can get about 30 db of
rejection on the offending band. Operationally, you simply switch
to the offending band and tune the device for minimum band noise,
and put little notation marks on it for reference during the
contest.
-----------------------------------------------------------
RG-142BU which is that fancy, teflon-on-the-outside, the-size-
of-RG58-but-takes-5KW stuff that you find at swapmeets. Using
small coax has it's drawbacks, however, becacause of it's higher
loss. One thing to remember about coax stubs is: The greater the
loss, the sloppier the skirts of the filter, and the shallower the
null as you sweep it. In other words, hardline makes fabulous coax
stub filters, and RG58 makes ones that only work so-so. RG213 or
RG8 (same thing, really) have been found by practical experience
to represent the best all around compromise.
-----------------------------------------------------------
Noise bridge: Start with the coax a little longer than you expect it
to be after you've measured it to a 1/4 wave including the velocity
factor, then you use this noise bridge gizmo and sweep the coax
with your receiver. Where the hiss goes away is where the stub is
cut for. It was one of those technical marvel things where
everyone else in the room dropped their jaws and gasped with
amazement.
-----------------------------------------------------------
We use 2 sets of ICE filters. They are cheap ($30) and work well.
there are some buts:
...1 physical filter per band= a lot of filter switching (we built all
of them in a box)
...everywhere I went, someone always blew up the 20m filter. get a
spare
...200w only.
... we tested them in a lab, they DO filter, but only if the SWR is
good. The higher the swr, the worse the filtering
-----------------------------------------------------------
A comical note. At a multi/multi I know well, they did not follow
my advise and insisted in making their own 2Kw band pass filters.
They tested the filters with the boxes still open and they
performed well. They filled the filters with foam and closed the
boxes. First minutes of the contest: everyone blew the filters up:
the foam did not dry!
-----------------------------------------------------------
1) All the stubs I have used are 1/4 wave in length...if you leave
the end open, the other end will appear to be closed. If the end is
closed, the other end will appear open. So for example, to put a
20m null on you 40 antenna, put on a 1/4 wave 20m stub, and leave
the end open. Or you can put a 1/4 wave stub on your 20m antenna,
and leave the end closed. One trick that I have used for open stubs
is to put a small air-variable cap on the end, and make the stub
just slightly shorter than 1/4 wave...then you can set the null
exactly by twiddling the cap while listening to the receiver.
2) Don't use RG58...the better the coax, the deeper nulls you will
get.
3) Doesn't matter...I'm currently using 75 ohm double-shielded RG-
11 that was made for computer networks.
4) Yes, that is what I do...put a T connector on top of the switch. Be
sure to account for the switch changing the length of the stub (use
something like a noise bridge to cut them to length).
5) My current setup...I have 4 1/4 wave stubs on a coax switch to
use with an all-band dipole. The stubs are closed on the end, so I
switch in the one that is 1/4 wave on the band I'm operating on.
There is some improvement, but not that impressive (my antennas
are very close together). We played around with some of the ICE
band filters at FD, and they seemed much more effective. Probably
would also be lighter to take filters than lots of coax stubs on a
dxpedition!
-----------------------------------------------------------
If you wanna really do it right, you need bandpass filters and
stubs!
-----------------------------------------------------------
1) Length is critical. Use a network analyser or other device to
tune the stubs. Stubs are very frequency sensitive... you'll need
one for SSB, and another for CW if you are trying to squeek out
evry last dB.
2) Power was not the problem. RG58/RG8X did not (in my case)
provide nearly the same attenuation per null as the RG8/213 did.
3) Suspect you can use 75 ohm. You will already have a small
impeadence bump when you tap for the stub.
4) Yep. Be careful, for the coax switch adds enough length to the
effective length of the stub to change the null outside your
desired band segment!
5) Good question. All depends if you want to do the filtering
before or after the amps. Amps are known for suprious emmisions.
The stubs will help cure that, Dunstar/ICE won't. If you buy new
RG213 for 160 thru 10, you come out about even with a set of
ICE's.
-----------------------------------------------------------
Thanks again to all for the good info. CU from PJ8Z in CQ WW SSB 94!
Mike N0BSH
n0bsh@aol.com
