Using a 144Mhz telescopic whip

[filtersw]

I live on a 14th floor apartment with no roof access. So my antenna options are limited. My PL-310ET radio works well, with no extraneous noise at the window. I picked up a cheap Diamond FH205 telescopic antenna from a local store. It is rated for 144Mhz.

The antenna is clamped against the window, and fed to my radio with a shielded coaxial cable. This allows me to listen at the desk, and not by the window.

The problem I have is that the antenna picks up a lot more noise compared to the radio's whip on "sub-optimal" day/night bands. Example: 31m band in the day is noisier with the FH205 antenna, and 22m - 19m is noisier at night. 31m band at night and 22-19m band in the day is excellent.

I confirmed it wasn't the coaxial because the radio is dead quiet when the antenna is disconnected. So I'm curious:

1. Is the noise caused an antenna not rated for below 30Mhz?
2. Are there special filters for the built-in whip that isn't present on antenna inputs?
3. Do I need some kind of balun/choke transformer between the antenna and coaxial cable?

I'm pretty new to shortwave listening so any advice will be appreciated.

 

[SpectreOZ]

You might have better results with simply stringing up a random length of wire around the top of your walls as man made noise is predominantly worse with vertically polarised antennas, alternatively if you feel adventurous... you could build a magnetic loop antenna which has the added bonus of being able to be positioned towards the best signal reception (direction) along with tunable resonance

 

[filtersw]

Thanks for the reply. I've actually tried a random wire antenna, hung around the window, or stuck out of the window, with some of it hanging down. The results were exactly the same as the FH205 telescopic antenna. Neither of them were better than the built-in whip antenna stuck out of the window.

That's why I'm curious if the "144Mhz" HAM antenna is worse because its length is in the wrong frequency band, or if built-in whip antenna may have better filter circuits in the radio.

This problem isn't a deal breaker, but I'm just trying to "get it right" on my next try without spending too much money. Right now, I'm considering the MFJ-1899T.

 

[ka3jjz]

Verticals by their very nature are more susceptible to noise pickup - so in an apartment, that's not always the best choice.

There's something else to consider here. Are the noise problems more prevalent when you are running off the mains, rather than on batteries? It's a very well known fact that these little portables come with wall warts with next to no shielding. They're a very common source of noise problems with these little radios. Sometimes choking them off with beads or a choke will cut the noise down, but not always.

There's really no filter that will knock the noise down entirely. HF in an apartment is a challenge in any case because you're going to hear every noise source in that building (there are MANY such possibilities - too many to list on a forum like this...).

If it were me, I would want to use the entire window as an antenna. See if you can get a small clip lead and attach it to the screen (if the screen is metal, it would make a decent antenna for that little radio). Another possibility is to use a loop, as was suggested before - if you have the parts and have some basic construction skills, this works pretty well for what it does...

The Carpet Loop -- antenna special on hard-core-dx.com

Mike

 

[nanZor]

Right now, I'm considering the MFJ-1899T.

That 1899t would be a sensible choice. Even though designed for the amateur bands, you have enough flexibility to change the taps as necessary for swl'ing with that 310 without possibly overloading it.

I use it with my Icom R20, although it is too heavy a load on the bnc connector, so I mate it up to an MFJ-332B mag mount - stationary that is with no wind! If you like, place the magmount on a bit larger steel plate, and attach radials to that if you like. The magmount is designed for vhf/uhf rubber ducks, but if you are a bit careful, the 1899t can stand on it's own.

Radials can be nothing - and your coax will become the only radial. Or, you can put the magmount on a small steel plate, and run some radials from that into whatever space that fits - fold them back if you have to. With the 310 receiver, absolute precision is not critical, so you run out as much radial as necessary, and use whatever tap position provides the best benefit. In cramped conditions, I prefer to fold back radials (leaving one end free of course) rather than running them back into the living quarters around walls as this exposes them to more noisemakers. On the lower bands, like 31 or 60 meters, I am forced to use only one folded-back radial running off to one side. Not the best, but you do what you have to.

Since the 1899T isn't terribly tall, you can fit it into many places, and there is no rule stating that it must be absolutely vertical in an apartment. Like Mike points out, noise can be an issue, but indoors you may want to mount place it in the position that nets you the least amount of noise, rather than aim solely for signal strength. You may have to get creative and hang it horizontally with fishing line and access to the taps - empirical creativity is a must indoors.

Running vertical in a household with low noise can be done - you just have to find the "hot spot" for it as if you were trying to find a space for an amplified whip. Sometimes you get lucky as I do. And sometimes you need to go further like choking the coaxial feedpoint, or merely moving the whip a foot or two one way or the other to get into the noise null.

You can test for least noise by utilizing your existing radio, running on batteries, holding the whip upright, and SLOWLY moving around the house - hopefully near a window. If your radio has ssb capability, use that as heterodynes will be easier to detect. Be sure to differentiate between open band-noise, and man-made noises. Also, some man-made noises may disappear for whatever reason during the times you are listening, so check at the appropriate listening times - ie, trying to listen to 17m at 3am is not even working from a propogation standpoint, so my neighbors garage light that puts noise there during the night isn't a concern.

Running from batteries only is a wise choice indoors - at least at first to cut down on the noise variables.

 

[filtersw]

Verticals by their very nature are more susceptible to noise pickup - so in an apartment, that's not always the best choice.

There's something else to consider here. Are the noise problems more prevalent when you are running off the mains, rather than on batteries? It's a very well known fact that these little portables come with wall warts with next to no shielding. They're a very common source of noise problems with these little radios. Sometimes choking them off with beads or a choke will cut the noise down, but not always.

Thanks for the suggestions. The antenna is mounted against an open window, with the rod pointed 45 degrees into the sky. I'm aware that mains may cause noise so I've been running the radio on battery and turned off everything else in the room, so far with no success.

I've read about using ferrite beads to choke noise. Online advices included rolling the coaxial cable a few times around the bead below the antenna. But if the coaxial is already shielded, will these beads be helpful?

You can test for least noise by utilizing your existing radio, running on batteries, holding the whip upright, and SLOWLY moving around the house - hopefully near a window. If your radio has ssb capability, use that as heterodynes will be easier to detect. Be sure to differentiate between open band-noise, and man-made noises. Also, some man-made noises may disappear for whatever reason during the times you are listening, so check at the appropriate listening times - ie, trying to listen to 17m at 3am is not even working from a propogation standpoint, so my neighbors garage light that puts noise there during the night isn't a concern.

Running from batteries only is a wise choice indoors - at least at first to cut down on the noise variables.

I think what I'm fighting against are open band-noises. I hear the same noise with the whip antenna pointed out of the window. The problem that I'm trying to solve is that these band-noises are much louder with a random wire or that Diamond antenna, overwhelming broadcasts. They are especially loud between 22m - 13m band.

But just to be sure, I'm going to try different rooms tonight to see if upstairs or downstairs neighbor maybe generating those noises.

 

[filtersw]

Problem summary

I experimented around the house today, with the radio on battery and these are my test results on the same station above 13,000kHz

1. Antenna mounted on window + radio at window sill + coaxial on window sill = broadcast with no noise
2. Radio on table + coaxial to window (antenna not connected) = silence
3. Radio on table + coaxial to window + antenna mounted on window = broadcast with loud open band noise


So #2 confirms that the coaxial is well shielded, but #3 seems to indicate that a complete circuit with the antenna causes the coaxial to be "part of the antenna" and pickup noise within my apartment.

I'm using RG58 C/U coaxial cable. Can I solve this problem with better shielded coaxial, like RG-6, or should I buy or install some ferrite bead choke?

 

[filtersw]

PROBLEM RESOLVED

It appears that "shielded noise" from the coaxial cable is radiating from the external BNC connector, to the antenna and back into the radio. I resolved the noise issue by shorting the coaxial core to the antenna core pin with crocodile clips.

This will be the workaround until I figure out a way link the coaxial to the antenna without having the shielded noise radiating back to the radio.

(Sorry for the thread spam, I can't edit old posts that were moderated)

 

[nanZor]

Glad you made some progress. I'm pretty sure your coax not having any real rf ground is part of the issue.

In fact, I pulled the 1899T out of the closet, attached the magmount to a steel pizza cooking plate with radials - the whole 9 yards. While I like it as a small portable adjustable antenna, day in / day out operations by adjusting the coil AND the whip could get tedious pretty quickly.

In addition, I had to choke the coax a LOT since there is no direct connection to the radials with a magmount, and that feedline common mode just wants to be part of the antenna. I can manage choking it, but that, along with constant tap and element tuning might drive one crazy unless they are doing short weekend / portable stunts.

So forgive me here since I overlooked operational convenience. It's fun to have but perhaps keep on trying to identify the cleanest areas of the house, and hang a wire in that low-noise (but not completely rf shielded!) zone.

 

[filtersw]

Glad you made some progress. I'm pretty sure your coax not having any real rf ground is part of the issue.

Yes, that's true. The water pipe is way too far from my room for a practical ground connection.

In fact, I pulled the 1899T out of the closet, attached the magmount to a steel pizza cooking plate with radials - the whole 9 yards. While I like it as a small portable adjustable antenna, day in / day out operations by adjusting the coil AND the whip could get tedious pretty quickly.

If I understand correctly, the 1899T is only effective with the coil jumper?

In addition, I had to choke the coax a LOT since there is no direct connection to the radials with a magmount, and that feedline common mode just wants to be part of the antenna. I can manage choking it, but that, along with constant tap and element tuning might drive one crazy unless they are doing short weekend / portable stunts.

Speaking of choke, the permanent solution to my problem was a short single core wire, with BNC on both ends, rolled 3 times through a large ferrite bead and placed between the coaxial and the antenna. The noise problem went away.

Short related question, is RG6 the best shielded coaxial cable? I know it's 75 ohm but I'm using it for receiving only.

 

[nanZor]

If I understand correctly, the 1899T is only effective with the coil jumper?

Basically yes - you have to move it for each band. If you don't use it at all, it is intended for 80/75 meters. Use the jumper from the 1st to 2nd position and now it does 40m. For swl, 41 meters is close enough. Thing is, it is a very narrowband antenna, so you have to move the jumper and tweak the whip length sometimes. Too much trouble for daily use.

Speaking of choke, the permanent solution to my problem was a short single core wire, with BNC on both ends, rolled 3 times through a large ferrite bead and placed between the coaxial and the antenna. The noise problem went away.

Hopefully that single wire wasn't part of the antenna itself - otherwise you just attenuated all your signal away and put the noise below the noise floor - which works, but is not ideal and similar to just using an attenuator.

Speaking of which, to choke my magmount's RG58 feedline near the mounts feedpoint, it took 4 rf chokes. I used a pack of MFJ 700C ferrites (for HF, not scanning). Made a two-turn loop about 6 inches in diameter, and snapped each choke over each double-turn. Now I could grab the feedline and not have it affect tuning - letting my real radials do the job which are not close to the living / operating quarters where much of my computer noise, etc comes from.

Short related question, is RG6 the best shielded coaxial cable? I know it's 75 ohm but I'm using it for receiving only.

It's fine. Just watch your bend-radius, as if they are wound / routed too tightly, say less than 6 inches diameter, the center conductor can migrate away from center and short out internally - especially in hot weather.