RadioReference on Facebook   RadioReference on Twitter   RadioReference Blog
 

Go Back   The RadioReference.com Forums > Topic Specific Forums > Antennas and Associated Hardware > Scanner / Receiver Antennas

Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1 (permalink)  
Old 03-25-2013, 3:37 PM
Completely Banned for the Greater Good
  Premium Subscriber
Premium Subscriber
 
Join Date: Feb 2013
Location: Hicksville, Long Island, NY
Posts: 1,431
Default low-band travel antenna...?

(I thought about placing this in the California forum - if a moderator believes this should be moved please do so. I only put it here because, while it is about CHP, it's a general antenna question that might have more general interest to others)

I’m considering monitoring ONLY CHP when I travel to the San Fran bay area/Los Angelos/Long Beach/San Diego areas.

I thought about getting a dedicated, professional quality low-band whip (like an Antennex) and replacing the center whip with a piece of thick insulated wire.

I’d cut the wire to a length that was centered in the 39-45MHz range, connect it to the base by utilizing the hex key socket and coiling-up the wire when I travel.

When I arrive to my hotel, I would put the antenna near a window with a small NMO mag-mount, uncoil the wire and tape it vertically against the window. Now I have an instant, dedicated low-band whip.

Will this work, or will I need a ground plane also? I was thinking those long metal A/C units that are located right under hotel windows would work, but if I need to add some wire "radials" I'm OK with that too!
Reply With Quote
Sponsored links
  #2 (permalink)  
Old 03-25-2013, 8:09 PM
Member
   
Join Date: May 2009
Posts: 1,685
Default

Sure, that will work. How well is another story.

A quarter wave low band antenna indoors with only a magmount on an air-conditioner or 2-radial system is practically guaranteed to be detuned to close object coupling, and have a skewed pattern due to common-mode currents on the coax.

But if signals are strong enough, that may be immaterial.

If it were me, I'd simplify it by using a smaller tuned/loaded antenna to reduce the close coupling, and just accept the common-mode current and make it work to my benefit.

A Radio Shack center loaded telescopic #20-006 when fully extended is resonant near 45 mhz. Instead of radials, just choke the existing coax feedline with at least 4 of the RS ferrites like the #273-105 clamp-ons, starting around 50 inches away from the bottom bnc coupler of the telescopic on the coax. That and some blue painter's tape for the temporary attachment to the window.

The RS #20-551 back-of-set with 90-degree bnc adapter could also be used. When fully extended it is resonant near 43 mhz, and would be a good candidate although you'd have a small 90 degree angle to deal with.

Since they are both center-loaded, they are smaller than the standard quarter-wave, and might be easier to deal with indoors or keep away from other metallic objects like window framing which will help keep the horizontal pattern somewhat reasonable.

Last edited by hertzian; 03-25-2013 at 8:15 PM..
Reply With Quote
  #3 (permalink)  
Old 03-25-2013, 9:16 PM
Member
   
Join Date: May 2009
Posts: 1,685
Default

Forgot to mention that the RS #20-551 back-of-set telescopic is actually about a foot longer than the #20-006, and would be my preference for low-band when used portable.

A foot doesn't sound like much, but it is still an improvement on low-band, and I'd just deal with the 90 degree angle with an adapter or something if I had to before attaching to the coax.
Reply With Quote
  #4 (permalink)  
Old 03-25-2013, 11:50 PM
Completely Banned for the Greater Good
  Premium Subscriber
Premium Subscriber
 
Join Date: Feb 2013
Location: Hicksville, Long Island, NY
Posts: 1,431
Default

Hertzian,

Thank you, I appreciate your well-thought-out and very technical posts.

I am familiar with ferrite beads but I've never used them in the manner you're describing. What do they accomplish in this case? Also, how far do the 2nd, 3rd and 4th bead need to be from the first bead?

Last edited by LIScanner101; 03-25-2013 at 11:52 PM..
Reply With Quote
  #5 (permalink)  
Old 03-26-2013, 3:29 AM
Member
   
Join Date: May 2009
Posts: 1,685
Default

The beads would be right next to each other.

Without a good ground, or good radials (almost impossible indoors on vhf-low anyway) the "other half" of the antenna is going to end up being the outside of the coax braid. To RF, there are two sides of the braid - an outside and an inside. RF travels along a very small skin-depth, unlike dc current which flows through the entire thing. Ie, to DC the braid appears to be just one tubular wire. To RF, the braid is TWO wires separated by the skin-depth. To be complete, to RF, coax is a 3-wire circuit when the center conductor is included.

To emulate a dipole more or less, with the "common-mode" of the outer skin-depth of the braid acting like a wire, we want to choke the braid near the quarter wavelength mark. Because we are dealing with the outer surface, and not the inner surface conductors, the typical velocity factor for coax is ignored and can choke near the usual calculated lengths. The black coax sheath velocity factor is actually about .9, but in this application, I don't think you need to get that critical.

In essence, this is like a sleeve-dipole, using the RS telescopic as the top half, and the choked coax braid as the bottom half choked a quarter-wave away from the feedpoint. Instead of a traditional real "sleeve" made of tubing, for portable use, we're just going to choke the common mode with ferrites. Of course you could stick to your original plan of using say 5-foot wire for the vertical part and not use the RS whip.

I suppose one could also wind a coax coil around a 3-4 inch pvc pipe to accomplish much the same, but it would not be as portable.

Choking in this manner is not 100% effective, but it will tame it somewhat, and allow you to place it away from other conducting objects much more easily than radials running around the window ledge.

Last edited by hertzian; 03-26-2013 at 3:44 AM..
Reply With Quote
Sponsored links
  #6 (permalink)  
Old 06-28-2013, 11:34 PM
Completely Banned for the Greater Good
  Premium Subscriber
Premium Subscriber
 
Join Date: Feb 2013
Location: Hicksville, Long Island, NY
Posts: 1,431
Default

Hertzian,

I wanted to let you know that I finally had a chance to try your ferrite choke idea and while it didn't seem to help my CHP low-band reception (more about that later) it DID quell an odd effect I saw while I was travelling in CA earlier this week.

While "camped" in my hotel in Millbrae (right next to SFO) I went ahead and attached a telescoping RS scanner antenna to the window. I used a suction-cup mount with a female BNC bolted to it, and a 20' length of RG-6/U quad shield, leading to my scanner on a nightstand.

Before I added the chokes I tried it as is. Low band was very poor, but other bands were pretty good. The strange thing I noticed was whenever I got anywhere near the coax or stepped on it, it greatly affected any signal I was locked on.

I then clipped on (5) of the exact RS chokes you recommended starting at 50" away from the bottom of the BNC mount. No change on low band, but now it made NO difference when I held, stepped-on or appproached the coax.

So it definitely cancelled-out any common-mode currents therefore causing less of the entire coax to act as "part of" the antenna.

This leaves me to focus on what I can do to improve my low-band reception.

Last edited by LIScanner101; 06-28-2013 at 11:38 PM..
Reply With Quote
  #7 (permalink)  
Old 06-28-2013, 11:50 PM
Completely Banned for the Greater Good
  Premium Subscriber
Premium Subscriber
 
Join Date: Feb 2013
Location: Hicksville, Long Island, NY
Posts: 1,431
Default

Almost forgot to mention:

I noticed that when I did a service search from 30MHz to 46MHz my scanner woild stop on "noise". No transmission, just static. Worse, it happened even if I cranked my squelch all the way up. Is this noise coming from my AC wall adapter? Would adding a few ferrite chokes on the power input cable help?
Reply With Quote
  #8 (permalink)  
Old 06-29-2013, 2:48 AM
Member
   
Join Date: May 2009
Posts: 1,685
Default

Glad to hear the ferrites helped tune-the-braid so to speak.

As for the wall adapter, you usually place the ferrite close to the output of the adapter, rather than at the input to the scanner. In some cases, this doesn't help since the interference is going right through the plastic case of the wall cube.

Note that even modern cigarette lighter dc adapters with selectable voltages are no longer simple transformers and caps, but small switching supplies too! This burned me before chasing down a problem not thinking that a cigarette lighter adapter could be the culprit. Arrghhh -- switchers everywhere.
Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 8:34 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
All information here is Copyright 2012 by RadioReference.com LLC and Lindsay C. Blanton III.Ad Management by RedTyger
Copyright 2011 by RadioReference.com LLC Privacy Policy  |  Terms and Conditions