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antenna splint
- Thread starter dotnmarty
- Start date
Marty-
The antenna is really a metal wire or ribbon inside the fiberglass sheath that holds it up and in position. The radio "pumps' an eletric charge on the metal part, sending out signal waves. A metal sheath around it to splint it would greatly interfere with the broadcasting of the waves, depending on how long it was. A three inch plastic or wooden splint shouldn't hurt your antenna too much, however.The metal part inside the antenna should be intact for good performance, as the length is "tuned" to the range of frequencies that the radio typically broadcasts.
However, in an emergency where there is no other alternative, a shortened antenna will work over limited distances, just keep the metal part that's left from being grounded out to anything.
Be careful with the fiberglass rod. The glass fibers are sometimes exposed and can be inserted into a person's fingers and skin quite easily, causing nasty itching and pain. I have clear coated with urethane and even wrapped with tape, a fiberglass antenna that was worn to keep it from displacing it's glass shards into someone .
If you have one that frequently abrades when trailering, a section of clear plastic tube (Tygon at the hardware store) can be placed where it usually rubs to stop the abrasion and problem exposure.
Joe.
The antenna is really a metal wire or ribbon inside the fiberglass sheath that holds it up and in position. The radio "pumps' an eletric charge on the metal part, sending out signal waves. A metal sheath around it to splint it would greatly interfere with the broadcasting of the waves, depending on how long it was. A three inch plastic or wooden splint shouldn't hurt your antenna too much, however.The metal part inside the antenna should be intact for good performance, as the length is "tuned" to the range of frequencies that the radio typically broadcasts.
However, in an emergency where there is no other alternative, a shortened antenna will work over limited distances, just keep the metal part that's left from being grounded out to anything.
Be careful with the fiberglass rod. The glass fibers are sometimes exposed and can be inserted into a person's fingers and skin quite easily, causing nasty itching and pain. I have clear coated with urethane and even wrapped with tape, a fiberglass antenna that was worn to keep it from displacing it's glass shards into someone .
If you have one that frequently abrades when trailering, a section of clear plastic tube (Tygon at the hardware store) can be placed where it usually rubs to stop the abrasion and problem exposure.
Joe.
Don't use the metal pipe. Often 1/2" CPVC pipe is the correct size to fit over the antenna--check it out. If the antenna is too big for the CPVC then the PVC will work. Slide the pipe over the antenna and pump in some sealant (silicone will work fine).
I would not use a shortened antenna or broken wire. The radio output transisters will shut down when there is a SWR mismatch--and the output may be very low. You can either carry an "emergency antenna" or make one--but it has to be a specific length of wire to have the proper SWR.
I would not use a shortened antenna or broken wire. The radio output transisters will shut down when there is a SWR mismatch--and the output may be very low. You can either carry an "emergency antenna" or make one--but it has to be a specific length of wire to have the proper SWR.
mikeporterinmd
New member
thataway":3hexd3fx said:Don't use the metal pipe. Often 1/2" CPVC pipe is the correct size to fit over the antenna--check it out. If the antenna is too big for the CPVC then the PVC will work. Slide the pipe over the antenna and pump in some sealant (silicone will work fine).
I would not use a shortened antenna or broken wire. The radio output transisters will shut down when there is a SWR mismatch--and the output may be very low. You can either carry an "emergency antenna" or make one--but it has to be a specific length of wire to have the proper SWR.
How does the connecting cable figure into the SWR? Whoever installed
my old antenna cut the wire off at about 2'. Did this damage the
radio? I'd like to know because I replace the radio and wouldn't
mind selling it, but not if it is likely to be damaged.
Mike
Captains Cat
New member
While the connecting cable has some to do with the Standing Wave Ratio (SWR), it's not much. If the radio works (transmits/receives) on high and low power, it's not "damaged". The shutdown mentioned is a temporary thing the output circuits do to protect themselves. When the high SWR condition goes away, they're back up and running.
Charlie
Charlie
Well, thanks to your help, she's up and running. But I am worried about 'next time'. Is there a recommended 'shorter' VHF antenna that won't overhang the roof when folded down? My mount is simple and on the hand rail. I don't want to move the whole thing down and have to drill holes. Thanks again.
journey on
New member
One can buy all lengths of VHF antennae, down to the short 3 ft. sailboat antennae. The tradeoff is gain (the taller put out twice as much electrical power: 6 dB vs 3 dB,) and height. I have a 8' plastic antenna and of course need to lay it down when we're trailering. To do it neatly, I have to remove the antenna from the mount, and then tie it to the handrails. I left enough of a service loop in the cable to be able to do this. Pain in the rear, but it works.
Boris
Boris
Boris-I only trailer the boat and since it is a 16 footer I'm gonna have overhang. I really don't want the extra nusiance of doing that disconnect every time. Also and probably more importantly, I don't want to poke a hole in my winter cover to avoid the stress on the fiberglass pole. I wonder how much practical difference 3db vs. 6 db gain really would make in my Puget Sound boating? Thanks
A
Anonymous
Guest
We have an 8 ft digital antenna , rail mounted on Huda Thunkits roof. When folded about 4 inches hangs off the back of the cabin. When trailering and when in winter storage we just slip a 6 inch piece of 3/4 pipe insulation over the antenna end. Using one of those smallest bungee cords, the one with a ball on the end, we can hold everything securely to the rail.
We are into our 2nd winter with this arrangement and it has endured numerous snow removals with out incident.
Mike on Huda Thunkit
We are into our 2nd winter with this arrangement and it has endured numerous snow removals with out incident.
Mike on Huda Thunkit
Agree that trimming down the coax to the antenna will make little if any difference in the SWR. Most antenna manufactures will say not to make the coax shorter than 3 feet. There is a little loss in Coax, but it is so little that not negiable.
For a short antenna, I used a 3 foot Metz SS whip base loaded on top of a 2 foot SS extension. This prevented the broken whips which overhung the cockpit or top.
I like the digital over the Shakespheare. I take my antenna analyzer to the store when I buy an new antenna, and find that the SWR seems slighter better on the Digital.
Bob Austin
For a short antenna, I used a 3 foot Metz SS whip base loaded on top of a 2 foot SS extension. This prevented the broken whips which overhung the cockpit or top.
I like the digital over the Shakespheare. I take my antenna analyzer to the store when I buy an new antenna, and find that the SWR seems slighter better on the Digital.
Bob Austin
thataway":1kwycfns said:For a short antenna, I used a 3 foot Metz SS whip base loaded on top of a 2 foot SS extension. This prevented the broken whips which overhung the cockpit or top.
Bob Austin
OK, that's good enough for me, a 3 foot Metz SS whip base etc. it will be. Thanks again.
Standing Wave Ratio (SWR) is a ratio of maximum voltage to minimum voltage along a feed line (Coax between the Transmitter and Antenna). Also... SWR involves how well the feed line impedance is matched to the antenna impedance; i.e. 50 ohm to 50 ohm. The latter should not be an issue if the antenna came with a factory attached coax feed line and the antenna is a marine antenna.
SWR is read by an inexpensive SWR meter in series between the transmitter and the antenna, with the meter being as close to the transmitter as possible.
With that being said, in a perfect world the SWR reading upon transmission should be 1:1. At a SWR of 1:1 100% of the RF signal sent from the transmitter is emitted by the antenna. Unfortunately, this ain't a perfect world and you'll get a SWR reading showing some of the RF power to the antenna is being reflected back to the transmitter along the feed line :sad. Newer radios have limiting circuits that reduce output power when the SWR goes above 2:1... this is a life saver for expensive radio rigs :teeth. I believe this mentioned in the thread. When this happens... the range of the radio rig is reduced in proportion to the reflected RF and the reduction in the output power of the radio.
So if this is the case, how is the SWR reading affected by an antenna? Simply put... it is the antenna length. The length of an antenna is based on the frequency (Hz) of the bandwith utilized. Antenna lengths are based on incriments of the frequency; i.e. 5/8 wavelength, 1/2 wavelength, 1/4 wavelength, ect. Normally, one would lengthen or shorten an antenna to match the corresponding desired wavelength by monitoring the SWR meter during transmissions. An antenna that is well matched to the output power and frequency of the transmitter is said to be tuned and resonant at that frequency. Not much RF signal is reflected back down the feed line and thus the SWR is low.
Fiberglass antennas are factory pretuned to the frequency of the corresponding radio, such as the 11 meter CB band and the 10 and 12 meter Ham bands. You can use a 10 meter antenna on an 11 meter frequency, however, that could be hard on the transmitter and besides the performance (ability to be heard beyond one's shadow) will certainly suffer.
In your case... I'd suggest buying or borrowing an SWR meter and a short PL-259 jumper coax to measure SWR after the you do the CPVC or PVC splint antenna fix. The reason is... a high SWR is an indication your transmission range may be severely limited.
I have witnessed SWR readings on marine radios so high the transmissions were not making it across a small marina... :sad. One in particular was a 8' fiberglass with the last foot at the top duct taped on. At 25 watts radio output the feed line would heat up before the radio's SWR limit switch kicked in... I don't think that guy's RF signal; made it past his stern less than 20' away.... :shock:
Hope this helps.... Butch KB30JO
SWR is read by an inexpensive SWR meter in series between the transmitter and the antenna, with the meter being as close to the transmitter as possible.
With that being said, in a perfect world the SWR reading upon transmission should be 1:1. At a SWR of 1:1 100% of the RF signal sent from the transmitter is emitted by the antenna. Unfortunately, this ain't a perfect world and you'll get a SWR reading showing some of the RF power to the antenna is being reflected back to the transmitter along the feed line :sad. Newer radios have limiting circuits that reduce output power when the SWR goes above 2:1... this is a life saver for expensive radio rigs :teeth. I believe this mentioned in the thread. When this happens... the range of the radio rig is reduced in proportion to the reflected RF and the reduction in the output power of the radio.
So if this is the case, how is the SWR reading affected by an antenna? Simply put... it is the antenna length. The length of an antenna is based on the frequency (Hz) of the bandwith utilized. Antenna lengths are based on incriments of the frequency; i.e. 5/8 wavelength, 1/2 wavelength, 1/4 wavelength, ect. Normally, one would lengthen or shorten an antenna to match the corresponding desired wavelength by monitoring the SWR meter during transmissions. An antenna that is well matched to the output power and frequency of the transmitter is said to be tuned and resonant at that frequency. Not much RF signal is reflected back down the feed line and thus the SWR is low.
Fiberglass antennas are factory pretuned to the frequency of the corresponding radio, such as the 11 meter CB band and the 10 and 12 meter Ham bands. You can use a 10 meter antenna on an 11 meter frequency, however, that could be hard on the transmitter and besides the performance (ability to be heard beyond one's shadow) will certainly suffer.
In your case... I'd suggest buying or borrowing an SWR meter and a short PL-259 jumper coax to measure SWR after the you do the CPVC or PVC splint antenna fix. The reason is... a high SWR is an indication your transmission range may be severely limited.
I have witnessed SWR readings on marine radios so high the transmissions were not making it across a small marina... :sad. One in particular was a 8' fiberglass with the last foot at the top duct taped on. At 25 watts radio output the feed line would heat up before the radio's SWR limit switch kicked in... I don't think that guy's RF signal; made it past his stern less than 20' away.... :shock:
Hope this helps.... Butch KB30JO
OK Butch- I'll get the SWR meter and the jumper coax. I'll try Radio Shack. I checked on the Metz 3 foot antenna and I think it's too expensive for me. West Marine has a 4 footer for about $100 which I will get if my reading is above 2:1. Thanks for taking the time to guide me. Marty