Batboard

I've never done anything with a 1/4 wave stub (notch) filter before.
What's the best physical way to construct this? I've seen hardline, coax and copper tubing used to build them before.

Is it a "true" 1/4 wave or a close calculation?

The frequency of the radio is RX 159.2, TX 153.7

This radio experiences interference from AM radio and also (possibly) a Nexthell site. The other reason is redundant lightining protection in addition to the Polyphaser.

My two bits worth:
The 1/4 wl stub may be of some use in lightning protection, since it is a DC short.
I doubt that it will be of much use for the AM station (.55- 1.6MEG), or
Nextel at 851-865MEG.

AM station problems are usuially due to detection in a device in your chassis. I have spent many hours determining where to place by pass caps, aluminum foil, ferrite beads.
This can be a real problem if the AM station is very close to your site.
Your equipment could be part of the conterpoise of the AM antenna.
A DC ground is not necessarily an RF ground.
Ground wires have impedance (Z) if they are long enough to be at resonant frequency they will do no good in eliminating interference.

The Nextel problem is so far out of band for your system I cant guess how it is getting in.
I presume that you have a band pass type duplexer.
If you are using a notch type duplexer, try putting a pass cavity in the receive line.

In trouble shooting these problems it is necessary to determine exactly how the offending signal is effecting your equipment.

The Nextel site on this building is just something we noticed the other day. We all know how clean Nextel is! It is possible that it may be combining with something else unknown. We didn't sweep the site yet.

This is just a base radio (M1225) installation.

The AM radio just started coming in about a year ago. It currently has 9913 coax (120 feet) with an unknown fiberglas antenna. There is no ground system at all. We will be installing a 2-bay folded dipole, LMR600 and a grounding system with Polyphaser and 1/4 wave stub. I understand that the 1/4 wave might not do much or anything at all with the interference, but we want to do it anyway with everything else new. As for the AM station, it is approx 7-10 miles away which makes us think that we have an intermod or combining situation- especially with 30 year old coax, antenna and no grounding and years of corrosion and water.

We're also considering ferrites on all leads on the equipment and the audio system in the building.

This setup has been altered over and ovre for years and is finally getting redone now. There are still many "unknowns" since we really didn't get into it yet. One other note- there is another VHF antenna on this building for the PD, but it really never sees TX duty.

Try this:

Calculcate the length required in "free space"

The length of the stub is measured from the center of the T to the tip of the short. So, the length includes the connector and the T. Subtract these from the free space length. This is the length needed for the coax in wavelengths.

The length of the coax is shortened by the velocity factor of the dielectric. Take the length you've calculated for the coax and multiply it by the velocity factor. The result is the length the coax should be when your're done.

It's best to start with the stub a little long (lower in frequency) and trim it to the correct length by checking it on a spectrum analyzer and tracking generator. Don't attempt to just follow the forumla and then just plug in it without checking it.

The diameter of the coax is the Q of the filter. Don't use anything smaller than RG213/214.

Shouldn't the T with shorted coax be 1/2 wave long at the freq you are trying to suck out?

A "window" filter that only passes 150-160 might be a better choice.
And the AM problem may be overload to other circuits that may be better cured with grounding, torroids on the coax (ie coming down the outside of the coax)

See this thread for more info, links etc...
http://batboard.batlabs.com/viewtopic.p ... =coax+stub

Read here for more info;
http://www.wrightsaerials.tv/Resources/ ... erence.pdf

Looks like if you make it 1/4 wave long, it should be open at the end, and 1/2 wave should be shorted...

<paste>

The quarter-wave stub, or teesection, consists of nothing more than a shortlength of coax connected to a point anywherealong the downlead (fig 6). The junction has nosplitter or impedance matching, but should bewell made and compact. The other end of thetee-piece is open; it is not terminated by 75Ω orby a short circuit. The length of the tee-pieceneeds to be exactly a quarter of a wavelength.That’s a quarter-wave in cable, not in space.Radio waves travel at significantly less than thespeed of light when they are in cable. The ratiois the ‘velocity factor’ and varies from about0.60 to 0.80 in coax, depending on theconstruction of the cable.

Well, I'm thinking that since he wanted lightening protection that it has to be a shorted stub.

Given that I beleive the two choices for stubs are:

(2n-1)/4 shorted at the frequency to be passed (155MHz or so)

or

(2n)/4 shorted at the frequency to be notched (850MHz and 1MHz)

I'm tending to think that making stubs for Nextel and AM Broadcast would be painful. That said, I'd be tempted to make a stub that is shorted and passes the frequencies of the VHF station.

What's your thoughts?

My two bits worth:
We are attempting to write a prescription for a problem that is not well understood.

We have assumptions that the interference comes from an AM broadcast station 10 miles distant as well as a Nextel system just because it happens to be near by. The AM station would not be demodulated in the FM receiver, Nextel would sound like a "Hiss".

An R1225 repeater on a site? The specs specify not suitable for conjested sites. Does this station have the original reject duplexer?

How would this NOT have problems? There is very little in the way of a pre selector in the receiver, and no protection what ever in the duplexer.

No intermod protection for this station as well as any other nearby users.

Perhaps the problem is that the equipment is not well suited for this application.

There is very little shielding for the audio circuits in this station.

I have seen this sort of thing often. Originally the first repeater on the site seemed to work well, only to misteriously deteriorate over the years. Coincidently, many other stations migrated to the same general area, such as a hill top with out proper engineering.

Radio signals do not respect arbitrary boundrys, such as property lines.
When your station is connected to the antenna, it is also part of your "radio community" like it or not.

You will probaby need to find some one that understands site issues so proper filters, etc can be installed.

This may not be cheap, but far cheaper than guessing.

This isn't a repeater- it's a M1225 "base" station in a small fire department that's experiencing this problem.
The most likely cause of the interference is the coax and antenna from when Nixon was president in conjunction with no grounding at all. I thought I'd mention the Nextel site just since it is there, although probably not the source. The VHF PD antenna on the same building is mainly for RX use.

As for hiring somebody to re-engineer the site....unless they can do this for under $1000, forget it- it's a small FD.

Another note- the AM station comes though the phone system also.
The radio and phone both tie into a common 100W amplifier that feeds the station speakers. The phone ties in only for PA use.

My two bits worth:
Not being there I can only guess based on what little I know about your system.

You likely have two or more RFI (Radio Frequency Interference) problems, which have unique solutions.

While it is a good idea to replace old coax and antenna, you may notice you hear some of the interference much better than before.

You still have a low tier mobile (a polite way of saying "Cheap") connected to a site antenna with NO protection.

The AM broadcast station is likely being picked up on the leased telephone line, being detected in the base-emitter junction of the first amplifier, or the line driver, which is connected to the telephone line.

The Telephone Company may have line filters for eliminating AM radio interference in telephone equipment, a rather common problem.

A .01uf/200V ceramic capacitor connected across the telephone line may help, as well as the same value to ground on each line.

By ground, I mean a real earth ground, not the 3rd pin on the wall socket.
As I previously stated Ground may have RF present if you are very near the station, in which case grounding will make the problem worse.

Applying the same concept to the PA system, you may find that by passing the speaker leads, or the other inputs will cure the problem.

Perhaps you need shielded leads.
The AM station must be detected in a junction, it cannot be detected in a wire alone. The idea is to short the RF signal while allowing the normal signal to pass.

The ARRL has publications for the amateur community that you may find helpful in curing your problem.

The ground will be a #2 standed lead with two 8' copper clad rods and will also be common grounded to the radio case, Polyphaser, shorted stub, PA amp and phone system.

We will be installatin ferritre beads and per your advice, bypass caps.

The new antenna will be an all aluminum folder dipole with 1/2" Andrew hardline.

It's too bad they were talked into a CHEAP M1225 instead of even a Radius or TK790. We've never had luck with these radios for anything more than a school bus mobile.